Where in-game can we get an item/cast a spell that PREVENTS the mummy curse? Nowhere. We have a way to remove it, but no way to prevent it completely. This is out of line IMHO. Adding stuff to the store that you cannot get anywhere else is a bad idea. Aside from cosmetic items.
Beholder absorption gems, while cheesy, replicate an effect already in game. The immunity to mummy curse does not.
Store items should be for convenience, not for things you can't get in-game.