In your place I'd definitely take it back. If it doesn't feel right now, it never will (or that's my experience, anyway). I've been going through all sorts of contortions trying to make my frankenbike work well for me but nothing does any good. The bike's geometry and mine just don't match up. Which might be bearable if it wasn't my everyday transportation, but it is so it isn't.

Get a disinterested witness to what the website says ("30 day money back"), save a copy of the page to your hard disk, and you can hold them to it--via the bbb, the city consumer affairs dept, or even in court if need be. If they dig in their heels and point to the register sign, tell them pleasantly that you relied on the website and you think they should honor that clearly-stated commitment.