It's very unlikely that the process of shipping a chip, particularly in the middle of winter, would warp the chip. It takes extremes of heat or water to change the shape of a piece of clay or plastic. And yes, if you like the chip, you can probably flatten it yourself, but it does take extremes of heat and/or moisture to do it. I agree with Mark, if you want to try this repair, experiement of a cheap chip of similar construction until you can see what it takes. Too much heat and too much pressure and you chip will not only flatten but squash, get larger, and lose surface detail.

My advice is, if you got a good deal on a scarce chip from a seller who doesn't collect, keep the chip and either enjoy it as is, or prepare to learn a bit about flattening chips.