I'm posting this mostly because I need to post something to indicate this is an active account.
But as it happens, I'm pretty familiar with Zazzle product lines. They have a couple of products that would do very well at conventions and which are ideal for displaying crisp cartoon-style art. I also think they would do well online. While the profit margins are not as good as buying your own and producing it yourself, there are no issues with inventory, so I think it's worth your attention. It's also a perfect way to do limited-edition prints, posters, etc.
Zazzle is not the only firm that does this, of course, but they are the one I'm most familiar with, and they do offer some unique things.
Here are the things I believe would do best at cons, as limited edition, dated loot.
This is the format of my dreams in terms of having my artwork reproduced. It's just freakin' beautiful.
A binder with WLP art on it for collecting originals and signed prints! Cool idea? It certainly means that even if you just break even on them, people are walking around the con putting everyone else's art into a WLP binder.
...well, it's a convention.
Other products I'd suggest exploring:
The embroidered products work VERY well with line art, but they require a setup/conversion fee. After that cost is recouped, though, there's no further investment other than marketing.
Card templates. I think a line of WLP character occasion cards is a natural idea. They can be set up as templates, so that people can customize them online, but with good generic defaults, you can certainly have a variety for cons.
Calendars. Their calendar layout is the most flexible I've seen. It doesn't even have to be an actual calendar. It could be an art portfolio. That might work rather well for y'all.
Zazzle has no problem with R-rated material, if it's labeled appropriately, and they are reliable and reasonable to deal with.