For Minecraft 1.13-1.19 use this version and this Signshop.

This is the beta release of SignshopMysteryBox feedback would be
greatly appreciated. This plugin requires Signshop v4.0.0 or higher.

Just like other Signshops you create the shop by putting the shop name on the top line of a sign and the price on the bottom line. But for mystery boxes you also put on the second line how many items the player should get for each transaction. The third line can say whatever you want.

Then you fill the chest with any items you want. The chest can be restocked anytime and with anything and is considered "in stock" as long as enough items are in the chest to fulfill a transaction.

You can create a [MysteryBox] which will take items from the chest when they are bought.

Or you can create an [iMysteryBox] which will randomly choose the amount of items from the box but will not actually remove them. Unlike other infinite shops you cannot destroy the chest.

Once you have the chest and sign made you you punch the chest with redstone and then punch the sign with the redstone to activate the shop.