This topic kind of cross lists with Post offices as well as every post office has a blue box, while there are blue boxes elsewhere, about half your waymarks for this category are already covered. Personally, I don't think it is all that interesting, I'd rather you have a waymark category for non-standard USPS boxes. (I saw SteveHerrick put up a R2D2 USPS box once which I thought was cool in the star wars category).