When I first started Shipping comics to buyers, the post office asked me how I wanted to ship. I said I didn't know. They asked me what was inside and I said "comics". They said ,"then you can go media mail, it's the cheapest".
So I've used Media mail ever since, Unless it's overseas obviously. If shipping and handling amount calculated reaches a certain amount or if the buyer requests it, I'll ship Priority Mail.
My local post office knows what I'm shipping and it's never been a problem.
Then when I was at Chicago Con last summer, I spoke with another dealer (who I buy from often) who lives in Canada, ships from New York. He had problems there. The first post office asked him what he was shipping, he said "comics - and I'd like Media Mail". The post office said he couldn't ship that way because comics contained advertising. So they went to a second post office and they asked him what was in the boxes and this time they said "Magazines" and they said media mail would be fine. This seemed odd to me, since Magazines contain far more advertising than comics do.
Personally I'm going to ship by Media Mail until they put a stop to it. I'll move to another location and say that they're books. A gray area I know, but people have a right to save money and it's worth the risk. If the advertisement situation becomes an issue, one could always argue that the % is so small that it shouldn't matter.