Which function handles membership level selection in Pro Site checkout page?

Hi all,

Needless to say, no matter how professional and modern your website design is, the WPMUDEV Pro Site checkout page will definitely destroy it. The pricing tables are ugly. They are not compatible with almost all the themes I've tested (8 themes), and look horribly miserable.

With that being said, I was trying to create my own pricing table. The only function I'm having is to attach events to that button so the correct level of the subscription will be sent to Paypal for payment. What event/url should I attach to those button to have it done?

Pseudo code:

<button onclick="javascript: processPayment(1);">Buy Level 1</button>

Please advise,