Adding a payment gateway
s2Member integrates with PayPal Standard, and the s2Member Pro add-on adds a few other payment gateways/services. See: http://s2member.com/features/
If you see that the payment gateway you want is supported by s2Member Pro and you don’t have it yet, go get it here: http://s2member.com/prices/
If you want s2Member to work with a payment gateway not listed there, find out if the payment gateway you want has an integration with s2Member, maybe they already did.
If they haven’t, you could ask them for it, create it yourself, or get a developer to do it for you. You can find freelancers in sites like jobs.wordpress.net, freelancer.com, elance.com or upwork.com.
These may be useful to create the integration:
WP Admin -› s2Member -› PayPal Options -› PayPal IPN Integration -> Proxy Key
WP Admin -› s2Member -› API / Scripting -› Remote Operations API
WP Admin -› s2Member -› API / Notifications
The Remote Operations API currently makes possible from a script outside of WordPress, to create a new user, authenticate an existing user, retrieve data from his account, update his account or delete it.
Or look at how one of the s2Member Pro payment gateways works and use it as a starting point for yours.
Keep in mind that if you don’t do a full integration, following the way the other ones have been done, you may have some limitations that you need to overcome or at least be aware of.
The one I know will happen if you use the PayPal IPN with proxy key, is that it’ll only work for new signups, not upgrades of existing users, because the user ID is not being referenced.
So it’s best to study an existing integration and emulate it to take advantage of all the features.