Mine is still "wavy", 2 days out of the box. I think some moderate heat to the Styrene with a sheet of plywood on top of it, followed by an elephant standing on the plywood SHOULD return it to its normal state.
Does having it rolled up warp Styrene?
I'll be able to work with the wavy Styrene as long as its straight enough to fit in my frames. *That* was my main concern with it.
As far as the Sintra goes, that I'm a little more upset about. Often I thermoform Sintra, but sometimes I just need a good, straight piece for "project X", and now that Sintra is all bendy. Again, hopefully some heat and pressing will fix it. Whether it does or not, the point is, I shouldn't HAVE to "fix" new material that is shipped to me. It should be packaged and shipped appropriately so I can use it "out of the box". Having to fix new material due to inproper packaging is irritating to say the least.
As far as other online vendors go, I used to order from Materiél Graphics in Chicago. They'd send me a 1"x1" piece of Sintra, if I wanted it. GREAT customer service, great prices, and low shipping. Unfortunately, they decided to stop selling to individuals and only sell to companies now.