The only consistent site I’ve used is CafePress, which has a lot of products to choose from but very little control over the design itself- you just gotta hope that whatever logo or art you’re putting on their thousands of products looks good without any futzing, because futzing is hella limited.
My CafePress stores:
I’ve tried using Teespring (and Stareable is currently using Bonfire to sell FORGET THE BOX TEESHIRTS EVERYONE GRAB YOURS BEFORE IT’S TOO LATE!!!) but I’m not a huge fan of having a time limit on merch (especially because you often have to completely recreate the listing to renew the design) even though the merch is higher quality by far. It’s just not a good fit for small time creators, and a pain to have to constantly monitor.