They borrow a credit card from the viewer and explain that it actually has no function, only the magnetic strip on it.
With these words, peel the magnetic strip off the card and stick it on the spectator's (or your own) forehead!
On the map itself, only a white field is visible.

Since the spectator can no longer do anything with his credit card, you make the black adhesive strip disappear and magically place it back on the credit card, which the spectator receives back unharmed!

Of course, the viewer's real credit card won't be damaged in the slightest: You'll receive the reusable gimmick and the necessary consumables, which you can easily reorder as needed, for example, from a stationery store or craft supply store.