Don't worry, it's normal. 

Some warping or cupping is normal, especially in thin and wide sheets. It's the nature of wood in this kind of sizing proportions. 

In use, it usually doesn't pose a problem. Baltic birch plywood is a utility material for making drawers and cabinets; projects in which the thinner material (1/4" and 1/8") is put to use in cabinet backs and drawer bottoms. They're captured within grooves or stapled or glued in place. Therefore the warping is inconsequential. 

However, we understand that you probably don't like it - or sometimes the warping is just too much to deal with. If that's the case, please contact us for a replacement.