Sometimes defects occur only after the first wash. In your case, since you haven't owned it for that long, I'm pretty sure you'll be able to return it. I remember reading somewhere that Threadless had replaced a shirt that started to peel at the design after the first wash.

I think you can download a return form on the site. Make sure you indicate the reason because they don't accept backwashed items unless it is a special case.

Shirts are made to last past the first wash, so your shirt is definitely defective.

I can't see what the difference is between shirt replacement and store credit in your case. With store credit, you can just buy the same shirt again, unless shipping is a big deal (which it can be). I think they will just replace it unless you specifically request refund or store credit (unless it is out-of-stock).

On the return form you can choose if you would like to get a new shirt or store credit.

Email Threadless to be sure they will take it back before you ship it.