(Qn 1) What are the options available? Is it just Authorize.net, or is Payflow Link (or Payflow Pro) also available,
Please see What payment gateways does s2Member® support?. Newer PayPal Pro accounts come with the Payflow API, which s2Member also supports. While s2Member does support Payflow Pro (the newer version of PayPal Pro), I don’t believe we currently support Payflow Link (which is similar to PayPal Express in that your customer gets transferred to PayPal’s website to make their payment, whereas with PayPal Pro/Payflow Pro, you accept payments directly on your own website).
(Qn 2) Are these available as default and supported by s2 (no custom integrations).
Yes, all of the gateways listed here are supported by s2Member without customizations.
(Qn 3) Furthermore, will transitioning from PayPal Standard w/ Pro Forms to Authorize.net be seamless (i.e., simple, similar logic), or is there a learning curve involved as they work differently.
Yes, the logic is very similar. If you want, you can enabled Authorize.net in Dashboard -› s2Member® -› Other Gateways and then explore the additional menus added. You can do this without affecting your existing PayPal configuration.
(Qn 4) Similarly, if Payflow is available, how is transition (seamless, simple, similar logic)? Please note that I have both Buy Now and recurring payment subscriptions on my website.
PayPal Payflow API is supported, as I mentioned earlier. It’s basically the newer version of PayPal Pro, so if you can’t get PayPal Pro, I doubt you’ll be able to get PayPal Payflow.
If you go from using a PayPal Express account to a PayPal Pro account, the only difference is that you can then use the s2Member Pro-Forms to accept payments directly on your website. You can still use the s2Member PayPal buttons feature.
(Qn 5) If I decide to keep the current setup (PayPal Std. w/ Pro-Forms) for the site launch (this week), and decided to change to Authorize.net, say, at a later date, say 3 months later. In that case, how will recurring subscriptions in the old setup work. Will s2 automatically transfer them, OR, will I have to keep both setups for awhile, and allow only new Authorize.net subscriptions OR will all old setup subscriptions be automatically cancelled. Also, similarly,
No, the subscriptions will not be transferred from one payment gateway to another. There is no automated way to do that.
However, you can use both PayPal and Authorize.net at the same time. So, if you decide to switch to Authorize.net later on, you can simply remove the PayPal buttons from your website and replace them with Authorize.net buttons. All your older members who signed up with PayPal will continue to have access. Their payments will still go through and as long as you leave your PayPal configuration in place, s2Member will continue to handle things like payment failures and cancellations. All your new subscribers will see your Authorize.net buttons and those new subscriptions will be handled by the new payment gateway.
If you want to “move” all your existing subscriptions from PayPal to Authorize.net, you’ll need to have your existing members sign up again through Authorize.net and then cancel their old PayPal subscriptions.
(Qn 6) how will the transition affect specific page/post purchases that still have time left before the end of their expiration terms.
Specific Post/Page purchases will not be affected at all. When someone purchases a Specific Post/Page, they receive an email with a special link that gives them access to the post/page for the duration you specified. Once they have that link, the payment gateway configuration on your site is irrelevant (you could even remove all payment gateway information from s2Member and those people who bought a Specific Post/Page would continue to have access).