Well, those are errors reported by Apache, not by PHP.
And, yes, you should fix those.
They're telling you that there are problems with the stuff you've added to your .htaccess file.
Zen Cart has some .htaccess files located in the various subfolders of the site, but not in the top folder. If you have a .htaccess file in the top of your site, then you've added that yourself somehow. And, if that file is causing errors it might be best to rename the file until you can come up with working content instead.
The most common reason why people add .htaccess files is to add URL rewrites to their site because they think search engines need it, when in fact they don't.
But that's somewhat unlikely to be causing your checkout page to be blank. The blank page you mention is caused by something wrong on your "fast and easy checkout" addon that you've added to your site. Either figure out the PHP errors it's causing, or disable/delete it from your site until you can set it up to work properly in a test-copy of your site that doesn't affect your live store.
.
Zen Cart - putting the dream of business ownership within reach of anyone!
Donate to: DrByte directly or to the Zen Cart team as a whole
Remember: Any code suggestions you see here are merely suggestions. You assume full responsibility for your use of any such suggestions, including any impact ANY alterations you make to your site may have on your PCI compliance.
Furthermore, any advice you see here about PCI matters is merely an opinion, and should not be relied upon as "official". Official PCI information should be obtained from the PCI Security Council directly or from one of their authorized Assessors.
