You do realized that bootstrapping requires not taking funding, which Teespring has taken plenty so far. Not to discredit their success, just clarifying.
For clarification, back in my day if the only funding you got was from founders and maybe an individual or two who believed in your idea [1], and consequently made it all the way to profitability, that was 'bootstrapping' the company.
Now get off my lawn sonny! :-)