I don't have that much experience to adjust PS3 settings regarding the payment gateways but you can definetely change the look of your shopping cart.
The easiest way to create your own index and maintemplate layouts and still keep the PS3 CSS files to adjust them when it's necessary.
After 40 hours work I was able to change the front-end layout completely incl. the site emails while the beatiful back-end still remains.