Grand Larceny and Using Someone Elses Credit Card

Thank you for your clear explanation. I will say all of my payments have been directly to the credit card company with the exception of the last 2 in the amount of $250 each. My sister wanted me to pay my Grandmother cash but I refused. I did pay her in money orders and made front and back copies of it as well as saved the receipts from the purchase and printed a copy of my bank statement showing where I purchased the money order. I also put on the memo line of the money order the credit card name and last 4 of the account number. Do you think I should just make the payments directly to the credit card like I was previously doing?

As far as signing for the card, I signed for it the way I would my own debit card. I never forged her name or anything of that nature.