Charity Shop Box Lot

My sister works part-time for a charity shop here in the Midwest United States. The shop manager recently gave her a box of stamps to show me after they had discussed that I collect stamps. I would like to give the shop a fair price for the box lot but can't decide how much would be fair. There are no albums or stock books. Just envelopes and glassines of world wide stamps sorted by country. Most appear to be from the 1960s to the early 2000s with the occasional earlier stamp. Most are off paper. There are just a few envelopes of U.S. stamps and a small handful of U.S. souvenir sheets. I normally collect U.S. stamps up to about 1970, but have been thinking about finding a few other countries or areas to collect now that I am retired. So this would be a good box to sort through on a rainy day. Anyone have an idea how I could relatively quickly determine a fair donation amount to give the shop?

Mostly different and not damaged? A penny a stamp, maybe two.

If you estimate that there might be 5,000 stamps, I'd give them $50. If you find something special in there, you can always decide to give them more.

My 2d.