Never heard of MAP4. Most CMC-made Verbatim 16x -R is MAPA, MAP6, or MAP7.
I’ve gotten better results from 16x CMC-made Verbs than Prodisc. The Prodisc has been inconsistent.
Media Mail via the postal service in the states would cost no more than $5 to send a 100-pack spindle of media. Very cheap.
Not all cans of air are created equal. Poorly made ones spill out liquid that can damage the disk surface. A light blowing of air should have no adverse effects on media. You also have to be careful never to tip a can of air too far when spraying, as liquid will be released instead of air. The can should remain mostly upright, and the object you are spraying can be adjusted so as to cover the maximum surface area.
The use of the phrase “dirt cheap” makes me wonder where you obtained the media from. Consumer-grade Verbatim in the states is no more expensive than any other brand of media, but when people start using certain phrases without giving the tell-tale indications that they are savvy shoppers, I suspect that something is wrong. Some people get things cheaply because they know how to shop and find deals. Other people get things cheaply because they have been ripped off. I can’t tell which situation applies here yet.