If you’ll make them pay, then instead of a free registration pro-form, you could use a Level 1 pro-form with Express Checkout as the payment option, that way they’ll enter their registration info in the pro-form and then on clicking the button they’ll be taken to PayPal to login and pay.
Ask PayPal to activate Express Checkout for you (which is free, and usually enabled by default). PayPal Express Checkout will give you API Credentials, and you’ll enter these under [hilite path]WP Admin -> s2Member -> PayPal Options[/hilite]. In the pro-form’s shortcode change the attribute [hilite mono]accept_via_paypal="paypal,visa,mastercard,amex,discover,maestro,solo"[/hilite] to just [hilite mono]accept_via_paypal="paypal"[/hilite].
Then you can customize where they get taken back to using the success attribute. [hilite path]Dashboard -› s2Member® -› PayPal® Pro Forms -› Custom Return URLs Upon Success[/hilite]
Login is not automatic after registration, you’ll need to apply a hack to achieve this. Some months ago I helped write one for this, but I don’t know how it’d work in the current release of WordPress, haven’t tested it lately. You’ll need to customize it a bit for your use, I guess.
Create this dir/file: [hilite mono]/wp-content/mu-plugins/s2hacks.php[/hilite]
[hilite pre_code]
[/hilite]
I hope that helps.