If you are speaking of the Save Changes button on the /index.php?main_page=checkout_one page when checking out, perhaps you might want to think of the Save Changes button as Insert Info instead.

Of course, you can change the button to whatever you wish but, regardless of the button's name, you have to get the individual's information in the system in order to process the sale.

I can't think of any cart that doesn't collect the three items needed; billing, shipping, payment method. OPC does that. And all without leaving the page.

I don't believe you can find a better product. You especially will not find one better documented or maintained.

We (myZenCartHost.com) would not create, update, or modify a site without making sure OPC was included.