Quick Question for the $2/day Plan. Refill Amounts.

Hello,

  I just have a quick question (and probably stupid) regarding the $2 a day plan on a prepaid ATT Go Phone.

 I have a card I can use to refill my account with. The increments are $15, $25, etc. Since Ive been short of cash lately Ive been using the $15.00 refill amount and everything is fine.

I am getting low and was wondering if I could add more funds on my account using less than $15.00 (I don't use my phone a lot). I usually refill at the local Dollar General, but I noticed if I attempt to refill online I can put a minimum of $10 on my account.

Before I try putting $10.00 on my account, I was just wondering if this is the correct minimum amount?? I would hate to put $10.00 on my account just to find out that I need to add more money.

I'd also be interested to know if this will push out my Account Balance Expiration Date another 30 days.

If anyone knows or can give me some insight on my question it would be really helpful.

Thanks in advance!!!!

PS. Don't know if it matters but I use the cheap ATT Samsung A157

PSS. Basically my question is what is the minimum amount I can put on my phone? Is $10 or is it $15?