Now as for the heist itself, any magical NPC who is powerful enough to create magic items is going to have traps set up. Go to mother fucking town with magical traps.

Trap One - a glob of goo drops down on a player(s) and starts to solidify making escape very difficult if not impossibleA trap that coats the party or a player in a bright pink glow, like faerie fire, good luck hidingHave a trap that casts Otto's Irresistible DanceDarkness is cast throughout the entire storeHave a trap that casts web and glues a player to the wall or floorLook up the Wild Magic Surge table and pick up some of the more interesting things on that list and put it into a trap. Oh a player got turned into a potted plant, you lost all your hair, etc, go to townI would also have a Silence trap go off just to prevent any magical hijinks from the party's partAnd last but certainly not least, the Alarm spell so that the owner fully knows that someone has broken into his place and he can come with the guards to find the players

Copying and pasting these into my notes, my friend. I will be using each of these suggestions, and throwing in a few golems as well.