Authorize.net is one of the most trusted payment gateway brands in the industry. Their reseller program (affiliate program) offers attractive buy rates and a residual payment commission structure. You can also earn additional revenue by selling other products on their platform.
Authorize.Net will pay you a $30 one-time fee for each referral you send, and then an additional $7.95 each month for the lifetime of the merchant account.
You can sign up here.