Just put all of the items on the store considered "unfair" in their own box that players can redeem in the wilderness (or where pvp is enabled), make it extremely hard for other players to get donation items from another user and make it extremely easy for the donors themselves to receive their items. That way, there's technically a chance for anyone to get those "unfair items". Just make sure you mention that "system/method of obtaining donor stuff" in your store (of course you'd keep out the "extremely hard" and "extremely easy" part). I've seen this method being used by many servers in order to circumvent the EULA and it worked.
