Ah, Mr. Fox, your reply has been an illumination... I had the feeling that the paper size and feed path were supposed to be chosen *prior* to accessing the driver settings, only every time I went to Page Setup in Photoshop, the paper sizes that were available to me were A4, Super A3 and several others, without any of the detailed Epson page sizes and border/less options. Since I was only printing on these two formats, it suited me fine.
However, upon reading your post, it was obvious that you were setting your page before accessing the driver settings, and I set out to have just one more look. Well, in case some other half-blind amateur searches the forums for this issue, here's the solution which escaped me for almost a week: the "Format For: " option in Page Setup should *not* say "Any printer", but should have the 3800 specifically selected. Since I have two printers on my machine, this was not the most obvious place to search (for me at least).
Thanks a lot for clearing that up for me.