The GTA Place Credit cards allow a user to pay for items with money stored in the bank without interfearing with the collecting of interest. However, there is a fee for paying like this. There is an intrest rate of all purchases with the cards, and the card will cost an ammount of money to buy; credit cards that cost less or are free will have higher intrest rates, but cards that are expensive will have a low interest rate. The cards will also increase intrest.
When you go to the bank, under the bank there will be links to buy credit cards. After you buy a credit card and go to buy something, there will be 2 options: Buy with on hand money, buy with credit card. As mentioned earlier, credit card transactions will take from your bank but don't interfear with the collect intrest.
So go enjoy buying with your bank.