Thanks for pointing out some nice resources. I'm finding answers to some questions, but not all. It would be nice one day to be able to add the purpose of each stamp to my albums.
I was poking around the USPS website and found a page with links to some historical data spreadsheets. Includes rates for everything including bulk mail, non-profit, etc. It's cumbersome and not user friendly, but does exist. I can't post links yet, so here is a pic. The "Domestic Rate History" link seems to be the most comprehensive.
Worldwide is certainly going to be much more of a headache, as most reources will be in the native language. I used to work for the Packard Humanities Institute creating a database of ancient Greek inscriptions on stone. To get the best text/reading for each inscription, we had to read all literature ever written about the stone, which usually meant that the original articles were all in the language of the country of origin for the stone. The joke between my boss and I was: "This stone was discovered in Albania - Guess it's time for us to learn to read Albanian!" So we did. I learned how to read (with very limited and specialized vocabulary) 14 European languages. I got this!