Zoe - Thanks for the help. Here's the page the PayPal return is worried about:
Spent some time following up on the SSL suggestion. Don't currently have private SSL cert for site, only shared SSL from my host. I believe the shared SSL will cause a certificate warning, so that may just add to worries. Would be great if I could just force this page inside S2 to SSL as in:
With s2Member installed, you can add a Custom Meta Field ( s2member_force_ssl = yes ) to any Post/Page. s2Member will buffer output on those special Posts/Pages, converting everything over to https:// for you automatically, and forcing those specific Posts/Pages to be viewed over a secure SSL connection; so long as your server supports the https protocol. Then maybe shared SSL would cover it.
In any case, thanks for the input. My launch is being held up but it seems there isn't a simple S2Member solution to this.
Andy