The plugin now have several updates:
Biggest one is possibility to work with ccaps. Means, you can use it to set different prorates when a member buys some ccap in same level. That feature was asked by a customer, then I add it to the plugin. But the "shadow side" is that setting of the plugin is more complicated. Still, it is logical, and since you know your site, must not be a problem. Anyway, I can set it for you, if you prefer.
We now have "by sum" type of pro-rate, it was planed long time ago, but was not build until now. Strange, but seems nobody uses it. Well, we have full set of pro-rate types, until someone invents something new :-)
Also a feature for developers. If you need the s2M's payment shortcode (like the one you get in s2M generator, at option pages) instead "compiled" HTML form, you can set the plugin to do that. See more info at plugin's page.