Manufacturing such products isn't our business, so it's hard for us to provide high quality merchandise like that. We do have some items available through a 3rd party here:
https://society6.com/esotericsoftware

The black travel mug is decent and I use it every day. I tried the stickers and they were not great. I'm not sure about the quality of the other items.

We setup the media for the items and so that website pays us when someone buys an item. However, they give us only $0.40 per item, so it's basically nothing. Definitely don't buy items hoping to support us, instead buy them if they are items you would like to have.

I expect there are many online stores that can take pictures and make them into a shirt/etc. We are happy to provide the media, if that would help. You can find official and high quality media here:
http://esotericsoftware.com/branding

If anyone knows which websites provide particularly high quality items, we'd be happy to set up the items similar to the site above so they are easier to acquire. In my experience commissioning Spine shirts to wear to events, the printing quality varies but the bigger problem I found is that the shirts are low quality with poor fitment. I have a couple in my closet I don't wear for that reason. It's be great to have a Spine shirt that fits well so I can actually wear it!

My shirt came! The printing seems fine. I don't know how to judge that really, but the colors are vivid and edges are good. Only time will tell how long until it flakes off. Logo placement is good. It has a scratchy tag but it was easy to pull off. It fits well, shows off the guns. I would actually wear this, except in hindsight black was not the best choice, the weather where I'm at is just Too Damned Hot for that. We need art that would work on a white shirt!

Actually after wearing the shirt, my Spine logo is too low (ES logo is OK). For anyone else doing this, this height works better:
Spine logo: http://n4te.com/x/8085-spine-12x16.png
ES logo: https://n4te.com/x/8002-esotericsoftware-12x16.png

I changed the links above so no one accidentally uses the too low version.

I thought so, but I just realized the Printful links don't have a way to purchase the product! What the hell. They seem to want us to integrate with their API or use a store like Ebay or Etsy. That is wacky and I don't want that, I just want to make it easy for people to get high quality branded products. I've emailed them.

OK, I had to turn on "merch share" for the links, now they have a purchase button -- yeesh.

When we find some time we'll brand a bunch of items and put a page on our site that links to the Printful pages.

BTW, sorry if my too low Spine logo messed up your shirt. Every time I look at it, it bothers me like sagged boobas.

Excellent! The gray is a better color than the black.