Full example

using System.Collections.Generic;
using BepInEx;
using BepInEx.Logging;
using HarmonyLib;
using MegaTree.Api;

namespace ExampleMegaTreeMod;

// NOTE FOR ANYONE COMING FROM SUPERMARKET SIMULATOR: this is BepInEx 5 and Mono.
// `BasePlugin`, `Load()`, `ClassInjector` and `Il2CppInterop` do not exist here.
// It is `BaseUnityPlugin` and `Awake()`, exactly like a normal Unity mod.
[BepInPlugin(Guid, "Example Mega Tree Mod", "1.0.0")]
[BepInDependency(MegaTreeApi.ModGuid)]   // without this, your Awake can run before Mega Tree's
public sealed class Plugin : BaseUnityPlugin
{
    public const string Guid = "ExampleMegaTreeMod";

    // Ids live in three places: the registration, the gate below, and the patch in
    // Effects.cs. Typing the same string three times is how a typo becomes a bug
    // that logs nothing, so they are constants.
    //
    // They are also the key in the save file. Never change one after release.
    public const string LoyalShoppers = "mtexample.loyal_shoppers";
    public const string GoldenHour = "mtexample.golden_hour";

    // The effect strength lives here, and BOTH the text the player reads and the
    // code in Effects.cs are derived from it. Write "3%" in one file and 0.02f in
    // the other and the player reads one number and gets another.
    public const float BonusPerLevel = 0.03f;

    internal static ManualLogSource Log;

    private void Awake()
    {
        Log = Logger;

        // One call, once. "Example" is the label of your sub-tab inside the Mods tab.
        var tree = MegaTreeApi.ForPlugin(Guid, "Example");

        // -- Node 1: the plain one --------------------------------------------
        var loyal = tree.Register(new UpgradeDefinition
        {
            Id = LoyalShoppers,
            DisplayName = "Loyal Shoppers",
            MaxLevel = 3,
            BaseCost = 1200,
            CostMultiplier = 1.8f,
            SortOrder = 0,
            Icon = UpgradeIcon.Builtin("shopping-cart"),

            // The percentage comes from the same constant the effect uses, so the
            // two can never drift apart.
            DescribeLevel = lvl => $"+{lvl * BonusPerLevel * 100:0}% on every sale",
        });

        // Register never throws - it runs inside YOUR Awake, and an exception here
        // would abort YOUR plugin over a typo. Checking the result is the only way
        // to find out something went wrong.
        if (!loyal.Success) Log.LogError($"Loyal Shoppers refused: {loyal.Result}");

        // -- Node 2: own art, a gate, a level requirement and a callback -------
        var golden = tree.Register(new UpgradeDefinition
        {
            Id = GoldenHour,
            DisplayName = "Golden Hour",
            MaxLevel = 2,
            BaseCost = 2500,
            CostMultiplier = 2f,
            SortOrder = 1,

            // Coloured artwork, flattened to a white silhouette for you.
            Icon = UpgradeIcon.FromPng(IconData.Heart, whiteSilhouette: true),

            DescribeLevel = lvl => lvl >= 2
                ? "Triples the Loyal Shoppers bonus"
                : "Doubles the Loyal Shoppers bonus",

            // The gate. Note what it does NOT need: any knowledge of the game's
            // internals. It asks the public API about another node, and that is
            // the whole dependency.
            IsUnlocked = () => MegaTreeApi.GetLevel(LoyalShoppers) > 0,

            // Always give a reason. A grey circle with a bare "Locked" is
            // indistinguishable from a bug.
            LockedReason = () => "Buy Loyal Shoppers first",

            // Optional, and null by default. The number is the game's own player
            // level - the same one that gates hiring and store growth.
            RequiredStoreLevel = lvl => lvl == 1 ? 8 : 15,

            // Fires on a purchase AND on a sell-back.
            OnLevelChanged = lvl => Log.LogInfo($"Golden Hour is now level {lvl}"),
        });

        if (!golden.Success) Log.LogError($"Golden Hour refused: {golden.Result}");

        // Translations are optional. Without this the English DisplayName is what
        // every language gets, and nothing warns you.
        //
        // "pt-BR" and "Portuguese" both work: the game names its languages, and
        // Mega Tree accepts the BCP-47 code too.
        tree.AddTranslations("pt-BR", new Dictionary<string, string>
        {
            [LoyalShoppers + ".name"] = "Clientes Fiéis",
            [GoldenHour + ".name"] = "Hora de Ouro",
        });

        // Nothing to do with Mega Tree: this installs the patch in Effects.cs.
        var harmony = new Harmony(Guid);
        harmony.PatchAll(typeof(Plugin).Assembly);

        // Worth logging what actually got patched. A patch that fails to install
        // is silent otherwise: the node works, the effect never fires, and there
        // is nothing in the log pointing at the cause.
        var nomes = new List<string>();
        foreach (var m in harmony.GetPatchedMethods())
            nomes.Add($"{m.DeclaringType?.Name}.{m.Name}");

        Log.LogInfo($"Example Mega Tree Mod loaded. Applied {nomes.Count} Harmony patch(es): {string.Join(", ", nomes)}");
    }
}

using HarmonyLib;
using MegaTree.Api;

namespace ExampleMegaTreeMod;

// THE EFFECT IS ALWAYS YOUR JOB.
//
// Mega Tree owns the node: the circle, the price, the gate, the save file, the
// hover card. It never knows what your upgrade does, and it deliberately has no
// way to find out. Everything below is ordinary Harmony against the game.
//
// WHY A POSTFIX AND NOT A PREFIX
//
// The bonus is paid as EXTRA MONEY after the sale, not as a bigger price on the
// sale itself. `OnPaymentFinished` has already told the statistics system what
// the profit was by the time it returns - rewriting the price underneath would
// make the end-of-day screen disagree with the bank balance.
[HarmonyPatch(typeof(CheckoutManager), nameof(CheckoutManager.OnPaymentFinished))]
internal static class Effects
{
    // NOTIFYING **IS** PAYING.
    //
    // `EventManager.NotifyEvent(ADD_SOFT_CURRENCY, x)` is not a notice about a
    // payment that already happened - the EconomyManager listens to that event
    // and IS what moves the balance.
    //
    // So you call it, and you call it ONCE. Calling AddSoftCurrency as well pays
    // the player twice, and that mistake is invisible until someone notices their
    // money going up faster than it should.
    private static void Postfix(float takenAmount)
    {
        int nivel = MegaTreeApi.GetLevel(Plugin.LoyalShoppers);
        if (nivel <= 0) return;

        // Golden Hour multiplies the line above instead of adding a second bonus.
        int ouro = MegaTreeApi.GetLevel(Plugin.GoldenHour);
        float fator = 1f + ouro;

        float bonus = takenAmount * nivel * Plugin.BonusPerLevel * fator;
        if (bonus <= 0.005f) return;

        EventManager.NotifyEvent(EconomyEvents.ADD_SOFT_CURRENCY, bonus);

        Plugin.Log.LogInfo($"[example] sale of ${takenAmount:0.00} paid ${bonus:0.00} extra " +
                           $"(loyal {nivel}, golden {ouro})");
    }
}

using System;

namespace ExampleMegaTreeMod;

// A loose png next to the dll is a file players lose, and then you get a bug
// report about a missing icon you cannot reproduce. Baking it into the assembly
// removes the whole class of problem.
internal static class IconData
{
    private static byte[] _bytes;

    // Decoded once, the first time somebody actually asks for it.
    public static byte[] Heart => _bytes ?? (_bytes = Convert.FromBase64String(Base64));

    private const string Base64 = "iVBORw0KGgoAAAANSUhEUgAAAIAAAA...";
}

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.1</TargetFramework>
    <AssemblyName>ExampleMegaTreeMod</AssemblyName>
    <LangVersion>latest</LangVersion>
    <ImplicitUsings>disable</ImplicitUsings>
    <CopyLocalLockFileAssemblies>false</CopyLocalLockFileAssemblies>
    <GameDir>C:\Program Files (x86)\Steam\steamapps\common\Megastore Simulator</GameDir>
    <Managed>$(GameDir)\Megastore Simulator_Data\Managed</Managed>
  </PropertyGroup>

  <ItemGroup>
    <Reference Include="MegaTree">
      <HintPath>$(GameDir)\BepInEx\plugins\rodopoulos\MegaTree.dll</HintPath>
      <Private>false</Private>
    </Reference>
    <Reference Include="BepInEx">
      <HintPath>$(GameDir)\BepInEx\core\BepInEx.dll</HintPath>
      <Private>false</Private>
    </Reference>
    <Reference Include="0Harmony">
      <HintPath>$(GameDir)\BepInEx\core\0Harmony.dll</HintPath>
      <Private>false</Private>
    </Reference>
    <Reference Include="UnityEngine">
      <HintPath>$(Managed)\UnityEngine.dll</HintPath>
      <Private>false</Private>
    </Reference>
    <Reference Include="UnityEngine.CoreModule">
      <HintPath>$(Managed)\UnityEngine.CoreModule.dll</HintPath>
      <Private>false</Private>
    </Reference>
    <Reference Include="Assembly-CSharp">
      <HintPath>$(Managed)\Assembly-CSharp.dll</HintPath>
      <Private>false</Private>
    </Reference>
  </ItemGroup>
</Project>