A couple things that might make the difference:

  - Conference swag shirts might be a lower quality/more shrinkable weave? I don't buy anything special.
  - I wash on medium or cold temp, never hot (except full loads of towels), and dry on medium. Everything always comes clean.

aside from that, I read the comments and I see that you are having no extra charge-up and this just feels like a passion project!

and I am grateful for that, Its actually really great and actually cheap.

Is there anything similar to this but for clocks for example? Niche, I know but I would love to have customized clocks perhaps via the same mechanism

Another suggestion I would love to have is if there could be a way that I can "gift" someone a t-shirt/credits ykwim

So I am thinking that if I ever do anything business related, I would love to give customers some good t-shirts with some text related to a business as a thank you for-example if they are using my service or as a giveaway,

But with your product I realize that I can also give them both the above option but also the fact that they can also customize it if they might not like it, I find that option of freedom for the end user somewhat even more respecting, and I can maybe give them many options to choose any one shirt to begin with, the possibilities are endless!

Great project actually. You should do a Show HN!