So this weekend I went to the local monthly stamp show here in the Troy, New York area. Between two dealers I was able to score 128 covers - all from the dollar box. Here's some samples below. I think I did well for buck-a-cover. Feel free to share your own dollar cover finds.
Postal envelope for use in Puerto Rico -

Not in the best condition, but that's okay -

This is not the usual dark blue color shade. Doubt it's the blue green, but possibly turquoise shade perhaps?

Unusual philatelic overprint on U.S. commemorative stamp -

I think this is the first time I've seen these U.S. postal cards postally used. Alas I couldn't find the airpost types.

First time I've seen an unused official postal envelope of this type.

Hmmmm sender's name ring a bell and there's a precancel under that cancel -

A place just up the road from me and it's still in service -

Nice American Indian advertising -

Interesting postmark - it wants to be a machine cancel so bad, but it's not.

Another item of official postal stationery that I liked.

..and probably my best find o' day -

Nice imperf revenue on a mining stock document. The mine is in upper Michigan and mined primary copper.