I don't have a lot of merch (at least I don't think I so) but I have a lot of quantities of merch, because it made sence to buy in bulk. I like having an inventory that doesn't need to restock anytime soon.
Most of my merch came as stretch goals from my Kickstarter this past summer.
I wanted to offer something I'd like to have if I were a fan of a comic, something practical, and something that can be shared, given to a friend- what I call referral art. I consider physically (floppies) merch, so here's my list:
The Goods (shown)
- issue 1
- issue 2
- coasters
- stickers
- 3 postcard designs

Promo materials I've made that I give away/leave at comic shops
- 3 bookmark designs (one is in the image)
- promo card stock flyer (not shown)
- my first set of coasters were awful material, but work as a leave behind at comic shops (not shown, but looks like the other coaster, just shiny).
I think with merch, it depends on what you intend to do with it.
For me, it is to reach an audience that does not read web comics and to encourage people to share the referral art. I leave floppies at shops and they typically won't take an issue 2 unless you have issue 1, so I try to keep a supply that allows me to do this. I've left shops with 5 or 10 issue 1's and when I check back on them, they're gone. A great sign, because they are more willing to take a chance on issue 2 (with issue 1 of course!)-- so having a bigger inventory allows me to do this with shops.
When I do a print run, I print 300. My interiors are in B&W and that helps keep the cost down. I'm pretty confident in selling them, but mostly it's to have them on hand when someone asks for it. I actually had to pull the floppies from my store because I wanted to be sure I have enough for 2 cons I intend to go to next year. I don't advertise them for this reason.
The kickstarter has taught me how to space out the rewards/stretch goals to make it more cost effective... but that is the only way I am going to be able to offer any sort of merch.