You cannot change anything now such as your attitude in being defensive, or for sure do not tamper with the numbers/letters because that could bust you. You said that she was a little playful in her answer , and if you leave well alone-she probaly will not go there and check it out.
I would buy something for her special and keep it somewhere like your car, or your home hidden somewhere, and then if she goes there and comes back and questions you then you can pull out the wrapped gift that you purchased and say that you were saving it birthday/anniversary or whatever date.
Then you can say that you bought it on Craiglist or ebay or somewhere and you had to pick it up-but the address written down must have been the incorrect directions or suite number.
I would say to say this really casually and do not be defensive at all.
I would also warn the girl at that place but they probaly have their own procedeurs for this sort of stuff.
And no, this never happenned to me !!
Good luck