why has the title been changed from "covers" (which is what they were) to "stamp" which is a less specific form of what they were? Why are we not just using the article title?
The article's talk page actually has a similar discussion over whether to call them stamps or covers in the title, so although they come down on your side its hardly a given.
Essentially they are envelopes with stamps and a marking over the stamps that prevents the envelope and stamps to be used.
Here's some examples of fancy covers:
https://store.usps.com/store/results/collector-s-zone-first-...