Mike McBride asks for some experiential information from Mac users:
My wife was doing a little on-line shopping today on her Mac and ran into a couple of sites that wouldn’t accept her credit card information. The transactions failed in both Firefox and Safari.
I’ve never had a problem with a shopping site taking my credit card, using Firefox or Safari. The problems that I do run across every now and then are sites that do browser & O/S checking badly (parts of Kaiser Permanente’s site refuse to display on anything other than IE5+/Win, even though their techs swear that they’re completely standards-compliant and browser-neutral) or that insist on using ActiveX components (these are getting fewer and fewer, fortunately.
Shopping sites though? Nope, nobody seems to have any problem taking my money. 
Mike, I’d be curious to know what sort of errors she was getting, and what sites they were if you don’t mind sharing.