David - it's dependent on where your buyer purchases them.
If they buy them on your AW, it depends. If you have a custom URL in the settings on your AW, then that's what shows. I have actually been directing traffic to artistname.pixels.com since it's the only secure version but my custom URL is still in the settings. And at least on the cards I've seen ordered through my AW, that custom URL is what is printed (which is fine because I forward that URL to the secure version). I would guess if you've never set a custom url, and they order from your artist website, then they'd get artistname.pixels.com on the back.
If they are bought on FAA then the FAA url to your artist profile is printed.
All that is of course subject to things not having changed in the past months. At one point it didn't matter where they were ordered from, the only thing I ever saw printed on the reverse was the FAA artist URL.
Mark
PS I've never ordered from Pixels main site, so I have no idea on that. I have one in front of me that was purchased on my artist website. About midway down, there's a logo with fineartamerica beside it and in small print under it fineartamerica.com Then further down near the bottom is three lines with the title of the art, my name, and the custom url to my artist website.