Pro sites plugin paypal (sandbox) checkout redirects to pro-site page

Hi there,

We've installed the prosites plugin on a multisite network, setup paypal checkout with a sandbox api account, configured a new level "Pro" (at $10/$30/$60 prices for 1/3/12 months).

We're unable to get the checkout button to send the user to paypal.

Theme is Twenty Eleven

Installed Plugins
Contact Form 7
MarketPress
Pro Sites (Formerly Supporter)
WordPress MU Domain Mapping

Apache .htaccess file

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule  ^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) $1 [L]
RewriteRule  ^[_0-9a-zA-Z-]+/(.*\.php)$ $1 [L]
RewriteRule . index.php [L]

Post params
psts-radio=1%3A1&period=1&level=1&pypl_checkout.x=72&pypl_checkout.y=17

Posting to /pro-site/?bid=<BLOG ID>

If we submit the form with WP-DEBUG set to true, this notice is printed
Notice: fopen(): Content-type not specified assuming application/x-www-form-urlencoded in /var/www/wp-includes/class-http.php on line 923

Any ideas how to resolve this?