The process is like this:

Sales page on your site >
Order page on our site (customizable on Edit pricing/delivery page under Order Page HTML) >
Upsell pages on our site (customizable on Edit Upsells page...you enter all the HTML on this page and choose a button) >
Thank you page (can be hosted on your site or on our site, edit this on Edit pricing/delivery page)

Additional keywords: checkout, upsell