Feathered placemat
 

well no pointers to speak of but you might try steam pressing the edges. not your feather which looks great by the way.

From the picture it looks as though the ripple goes all the way to the other edge. Could that have been the fold while it was stored? The thing about pillowcase quilting is that it is distorted by the quilting and is exceptionally hard to do without these kinds of problems. I have made many baby quilts with that method and this is not unusual. So, I try and use a very plain type of quilting and stop about two inches or more from the edges. Even with that, I sometimes just end up having to rotary cut the edges off and putting a binding on.

I wish I could give you more information, but, that is all I can describe to help. The method is great for lightly straight line quilted or tied quilts, but, not so much for heavily quilted ones.

I believe to avoid this is to do a lot of pinning with straight pins along the edge to distribute the fullness.

pretty feather cannot offer suggestions because i am having wonky borders after quilting. So I can say I feel your pain, enjoy the feathers.

I would say when you use the envelope method it will probably not lay flat with much quilting as it takes up fabric!!!

I have had this problem when pillow casing quilts. Fabric is never tight enough using this method so it is a draw back to perfection. I started ironing the edge of PC'd quilts before quilting so there is a nice flat seam around the edge. This helped a great deal.
peace