If you want to use your own join pages so that your customer never leave your site (which it sounds like you want to do), you should get your own merchant account and go through a gateway. You'll also save more money with your own merchant account. 5% compared to 15% with a 3rd party biller is a lot of money.
You should talk to Joe at
http://www.msusa.info
He can set you up with a merchant account pretty cheaply.