The shopping card can only be used at Gracie Grace. To get it, save 10000 Bells or more in your bank account. At Gracie Grace, you could use it to get two items a day for "free". The money will come out of your bank account. If you try to use the shopping card again to buy a third item (with Bells in your pocket), they will take your Bells.

You can get the Gold Card by saving a million bells or more in your bank account. It does the same the Shopping Card does, except you get a discount at Gracie's Grace.

If you never use your bank account (after getting the shopping cart), you can basically keep using the card to your heart's content and never pay it off.