I've never been there but when I was researching stamp stores in the Austin area, I kept getting leads on Stamp Antonio.
http://stampantonio.com/
They are hosting one of the amuseapalooza events, so they must be swell, right?
If you are going to be on the road between these two towns at all, I would really recommend you stop at Salado (north of Georgetown on I-35) at Stamp Salado. It is on Main Street, and it is WONDERFUL! It is small, a store that used to be a little house, but they have lots and lots and lots of stamps. I've got a pic of the outside of it on my blog, if you are interested.
If you are close at all, you would be happy you stopped at Stamp Salado!
HTH, good luck and have fun!