The Post Office advised me to mail my small (up to 9 x 12 paintings on panels in their Document Envelopes (cardboard on either side). They said mail goes through a machine and can be damaged, but these work great. I put them in the sealed bags first. Do any of you do anything different? Also, for small Gallery Wrapped Canvas, say 6 x 6 or 8 x 8, I have been wrapping in bubble wrap and then putting either pieces of foam board or cardboard on back or front. Any other suggestions, and are there certain boxes that you can get for these? Just noticed another post on this, that recommended boxes from Amazon, but kind of hate to order 100 boxes! Thanks in advance for any tips! 
I found the 9x12 Document Holder to not cover the sides well enough (good for smaller sizes though). They advised to just put in a Priority Mail Envelope marked fragile. I took it home, packed it up good, and mailed in a box. Cost a bit more than quoted, but felt better about the painting arriving safely! Thanks for responding Sarah :)2
I don’t ship flat paintings. But I will comment on small gallery wrapped canvas. I use USPS boxes that I order online (they do not carry this size in my PO) 121/2x3x151/4. These are free boxes and USPS will ship these free to your house! Very convenient. I always ship priority (this is not the higher ‘flat rate’) and it includes $50 insurance. I add more insurance, for the full value of the painting.
I ship my 8x10s in these priority boxes. I use bubble wrap, but DO NOT wrap a painting with this directly on the surface. It can stick to the painting, or cause an imprint. I use white tissue paper to wrap the painting first, then bubble wrap, then cardboard front and back. I have shipped hundreds of paintings this way and have had no problem. (knock on wood). Hope this helps!
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Thank you for you good suggestions about how to safely ship flat paintings. Very helpful!
Thanks so much Donna, this is very helpful info!
I use a company that does everything for me.