you need to hide the amulet in the shop, then later buy the wg potion and spike the shop owner's drink when checking her out.
Then tame the fairy, the fairy will make the shop owner slightly too fat to get in and she'll show you a back entrance to get into the shop no matter how big you get.
If you don't give the shop owner the amulet and wg potion you're softlocked out of the shop as far as I can tell.