I joined Teespring when it was still relatively small, and was their sixth engineer in the San Francisco office. I handled the implementation of a number of features, most important of which at the time was the seller-to-buyer messaging.
The next significant task involved taking charge of Teespring's localization initiative, adding the initial framework, then support for seven foreign languages. This evolved into working on the full globalization effort, allowing Teespring to function in non-US currencies.
I think this work set the stage for Teespring to move into the true global market that they've been aiming at from the start.