Hi Eben.
Also is there way to change the copy of submit form button ?
You can try this hack to change the text of the button: http://www.s2member.com/forums/topic/removing-your-css-styles-and-your-oops-text/#post-15849
Is there way to have one payment form for different custom capabilities and different prices (prices and capabilities are based on picked option)?
No, the shortcode doesn’t allow this yet.
One thing you can do is have a second form in the page, for the user to pick the custom capability he’ll buy, and submit it to apply the changes. This form would submit to the same page, and you’d need a bit of PHP to update the pro-form’s shortcode based on the user’s choice. This article explains how to use variables in the shortcode: http://www.primothemes.com/forums/viewtopic.php?f=36&t=1604
I hope it helps. :)