I agree. Ideally an extension never breaks a site. But if I were to make a store. The most robust thing in the store would be the checkout system. They’re leaving money on the table considering how little work it is to test the site with the few popular extensions. It’s basically like supporting a special payment method or exotic browser.
I’ve reported this to Teespring and all they reply with is to try another browser and clear my cookies. They won’t listen. They’re basically making Teespring unusable for the technical crowd.