Create paying member tier to Wordpress site using Verotel Flexpay API

I know how to do this with various plugins, but I'm looking for best possible solutions from those familiar with online commerce, PHP, adult credit card processing, and the Wordpress platform to code this directly into a Wordpress CMS using Verotel's API library, found here: [login to view URL] We already process with Verotel, but we're considering leaving that platform to work with something more limber and perhaps more easily integrated with the Wordpress CMS. At this point, I'm open to suggestion. I have already built, designed, and coded the site you'll be working on (some PHP functions were handled by other freelancers), so all designs and page styles are complete. The site is active and pulls in many members already. I'm just looking for a way to introduce them to a premium membership that will grant them access to other pages that free members won't have access to. It's that simple. Does this sound like something you're familiar with? If so, please get in touch and bid on this job. When you respond to this job offer, please verify that you can speak fluent English.