Hi,
I would like to suggest you have more details at the link snippet for S2 below.
wp-admin/admin.php?page=ws-plugin–s2member-paypal-ops
Something about whether people need to do anything more than set the appropriate setting in Paypal. As they have a page that asks me to download an encryption key for example and there is no mention of that in S2.
https://www.paypal.com/cgi-bin/customerprofileweb?cmd=_profile-website-cert
I am going to guess you have already done the coding so we don’t have to do this but it is only a guess and a little confusing not to have any info from you about it in the plugin.
-Gary.