PayPal Pro, PayFlow, DPRP, Express Checkout
There has been some confusion with PayPal's services, so I'm writing this quick article to help understand them better.
PayFlow Pro is not supported by s2Member Pro.
s2Member Pro supports PayPal Pro, which may be using the PayPal Pro API or the PayFlow API. The latter version is called PayPal Pro PayFlow Edition. http://s2member.com/kb-article/supported-paypal-account-types/
If you want to take payments on your site, make sure that you have PayPal Pro, not PayFlow Pro. Then check if you have the PayFlow Edition and, if so, include the PayFlow credentials in your s2Member integration. WP admin -› s2Member -› PayPal Options -› PayFlow Account Details
If you're selling a subscription and receiving payments on your site, then you'll need to upgrade your PayPal Pro account with the DPRP service. I think DPRP stands for "direct payment recurring payment".
In the PayFlow Edition it's called Recurring Billing. If you give a trial/initial term, even if it's a single payment, it's a subscription; trials are not possible with buy-now transactions.
Express Checkout can't be disabled in the s2Member pro-forms, it'll always be an option and the button has to be that one, can't be customized. If you're selling buy-now with Express Checkout, the user will not be required to login to a PayPal account to pay, but he will if you're selling a subscription.
If you have PayPal Pro PayFlow Edition and will sell subscriptions, you'll need to have Reference Transactions enabled for your account so Express Checkout works correctly.
I hope that helps with some of the common misunderstandings.