@trenty - I can't double check my work, but I have 4 contracts. I went contract by contract paying them, and I used 15 gift cards. On some if I had a $50 gift card and the remaining balance was 35.78, I entered 35.78 and it deducted that from the card and my owed amount went to zero.
As I paid off each contract the amount I owed for the month was reduced until it was totally eliminated.
I've never called for any payment, so I can't say what they would tell you.
Go to 'dues' on your dashboard and select pay dues and just read what it says. That's basically what I did and I didn't put much thought into it, so don't actually remember it all, just that I could have stopped entering cards at any time during the process and that would have left a remaining balance.