I think I got lucky. I had one sheet that had a slight curve on one corner, but that’s about it. I’ve kept mine in the box they came in since I won’t need them until my Glowforge shows up, so maybe that helped keep them flat.
My Proofgrade all arrived flat, but like every piece of plywood known to humans, once taken out of the box and unwrapped, it might tend to warp at least a small amount. I keep it all on a very flat surface with weights on top until needed. BTW: just because a surface looks flat doesn’t mean it is.
Interesting. A couple of us (at least) get these without any issue. Where are you? Perhaps there’s an environmental issue of high humidity or something. I’m in CT (northeast cold and usually low humidity).
Los Angeles - normally dry but mildly humid this week, so perhaps you are right. They came yesterday and definitely arrived bent. I’d estimate almost half are visibly not flat which makes me not confident about the whole box. Maybe I will return, time to find a local supplier.
I got the box of 20 and it was in pretty good shape. None were warped much. (Bought it in the winter though, our low humidity time of year, and brought it inside to acclimate for a while before trying to use it.)
We get like 70% humidity on average around here, so the local stuff is always pretty warped.
The first Proofgrade I got had a slightly warped sheet or two of the ply in it, but since then, I’ve left it tightly wrapped until I need it, and it tends to stay very flat.
I was able to call up a local building supplier shop, and a lumberyard/hardware store the next town over. They both will get me a sheets of 1/8 in (comes in 5 ft by 5 ft) Baltic birch-that is 15 planks of 12x20 in per sheet. For just about 15-16$ each sheet. One difference between the two is one charges an 8$ per transaction special order fee, and the charges for them to cut it down to 12x20 size differs.
YOU MAY HAVE OPTIONS LIKE THIS IN YOUR LOCAL AREA TOO! cheaper than Amazon, probably better quality, and you might even be helping a local business keep the lights on.
Most hardware stores carry baltic birch ply around here, and some of them will cut it down free, if you can find an associate and it’s not on a weekend.
But be careful of the quality. It varies, it varies, it varies. Some of it is solid glue, and you simply can’t cut through it. Multiple passes, incomplete cuts, knots, differences in density for the facings and the fill, charring, warping…it is hugely frustrating.
but it makes me extra 
about 
