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.
Running non-compliant extensions is your right, but the results don't need to be supported. I've run into this situation a number of times, although not with adblock. My advice has been to disable the extensions, or use a different browser without them.
You're leaving out the option of "not using the website."
Let's face it, nothing for sale on TeeSpring is essential. Just thank them for putting a roadblock on your silly impulse buy.
Compliant to who/what exactly ? Extensions are executed in a user agent, ie a software acting on behalf of a user based on the settings they prefer. If anything it's the website that's non-compliant with the user's choice.
It's your choice to use things that don't conform, such as IE11, but I wouldn't assume it's the server's problem.