I do feel pretty sheepish, though. I wrote it up on perlmonks before I got the solution, and now I've wasted space on perlmonks.
It does exactly what it's supposed to - on the first visit to the singles page, you get a text box and an explanation. If you type something in, you get all the cards that start with that letter combination. If you don't, you get the explanation again.
I'm adding the chunks now that deal with the checkbox info - if something's checked on the page and you send the page through (I'm not making it button-value dependant because some older browsers don't return button values), it's going to assume you want to put those things in your cart (you can always remove them later). Then it returns a the singles page with whatever's in the search box; this way, you can browse to the e section, click a bunch of things, put them in your cart, and browse to whatever new search term you put in at the top at the same time.
I hope this thing is easier to navigate than it is to describe. From my description it sounds like crap, and I'd really rather not build crap if I can avoid it.
Of course, this guy's going to be happy as a pig in $hit if I can get it to work in the next two weeks at all. He really isn't choosy at this point; he's a startup. So am I.
Okay - need to get back to it now. Otherwise I'll lose track of where I was going with the code.