Novel about people so average/boring that they are effectively invisible

This was a novel I read over 20 years ago. Unsure of the publication date, I think it was then current (late 1990s). The protagonist is a young man, a boring one with a boring office job… girlfriend leaves him because of him being boring. He comes to realize that no one notices him when he sees an older man like him waltz out of a convenience store with a case of beer unnoticed. He starts experimenting, discovering that he can get away with nearly any crime.

Eventually he reunites with his girlfriend, who has the same superpower. Things quickly get really weird (as if they weren’t already) when him and just a couple of others are actually exceptionally boring and become invisible even to the others like themselves. Something about the Great God Pan at the end (an Arthur Machen thing, if I’m not mistake).