When they open the doors you should be there ready to buy the shoes. Have an already typewritten letter explaining what happened. (just in case) Be courteous and if someone gets in your face and pulls a power trip just be contrite and agreeable.
The idea is to pay for the shoes quickly and quietly and have the receipt ready. Just go a cashier and say "I inadvertently walked out with these shoes yesterday." Don't go to the service desk.
After you have the receipt then go to the service desk and explain what happened.

