I was setting up an account for a friend recently and was struck by what I see as an oddity in your system.
My friend only receives the odd call sends SMS messages; she tops up up ten dollars each month and snaps up your great SMS package.
I thought it would be an excellent idea to set her up with a monthly automatic top up so she could set the pack to renew each month and the top up each month so she'd be able to set and forget it.
Unfortunately, the minimum top up for your site is twenty dollars. I can understand this, credit merchants are bastards :P
What I found odd was the lack of flexibility in your regular top up system. A perfect work around would be allowing perhaps a single month, two months or six month range. This would allow your customers the ability to top up once every 'X' months rather than every month.
As it is currently, none of the people using your SMS package alone can also make use of your automatic top up system; being that they would top up twice the minimum amount each month. This seems like a significant oversight being that you are the market leader when it comes to value SMS packages and must pull quite a portion of the market due to that.