I love my P800. It is an outstanding printer, in most respects.
Alas, paper handling is the one area where it not only failed to improve on its 3880 predecessor - but in fact is decidedly inferior.
By all means try the auto sheet feeder. The challenge with that option is that most fine art-type media will have those paper options grayed out and un-selectable. Epson, in their infinite wisdom, decided that all fine art sheet media should only be printed via the manual front feed. Mostly, that works okay. Especially if your media is perfectly flat. But using the front feed requires sliding the paper into a couple-millimeter-wide channel back into the bowels of the machine, where you can't see it. If your media is not perfectly flat it hangs up and you have to fiddle with it. It's not hard. And it typically doesn't take more than a few seconds to fix, but it is, IMO, a design weakness.
The other issue you might be having is the size of the cards might be smaller than the front feed accepts.
Best of luck!