I use Threadless:
https://www.threadless.com/
They had the better pricing tool I've seen from the services I used (TeaSpring being the primary other one). They also have an actual transaction log with details in case you need to talk to one of your members about a purchase, or if you offer rewards for purchasing products (I do). They also have frequent free shipping offers, including to Europe.
I chose them for options, the fact that I retained a lot of control over our store, and also could control the percentage I make off of our products.
A few things... There is currently no API, and it's been discussed by them for several years without showing anything. I have a rudimentary scraper made, and will also probably just have it expanded upon and made to work with Siropu's Ad Manager.
The quality and shipping is
generally good, however there have been a few screw ups, but their support has always dealt with things quickly.
A few members had products with printing issues, and they were replaced for free by Threadless without us having to be involved (better than when I dealt with other services).
One member had a weird issue with shipping where it got lost and they never received it, but they were then sent replacements as well as given credit for a additional purchase (does not remove income from us).
Their shirt prints are a bit finicky... Like all printed shirts, you should only wash in cold water, however I have seen that when they're not washed in cold water they can quickly degrade. They can also degrade quickly if they're worn a ton (my girlfriend wears one of the shirts almost every week, for the past year, and it's faded badly).
Overall I'm personally satisfied with them, and they offer much more detailed guides for how to make products for them which was something we had an issue with other stores. If you don't need a large amount of control, you can probably get by with other services.