If you mess up in scratching a gift card (tear the number, scratch too hard and invalidate it) - call the headquarters of the store the card is for. One example is Amazon - in making their gifts cards more secure so scammers can't use the cards, they made it more difficult for both the outlet selling the card, and the consumer, to get all the entire string of code that validates the card. The entire end of the string was torn off when my husband was getting the scratch off part from the gift card. I called and they told me to give the 16-digit card number at the bottom of the card, and whatever code pieces I could make out. The ones in the middle of the string are the most important and validate the card - then loaded the funds into my Amazon account. When in doubt - call and ask (or email if that's the only way to contact).
Gift Card ScamP.R.