Stamp Value Still On Envelope?

I have quite a few that are presidential covers. I have a few photos of the first "stacks" that I will post soon. Also some questionable airmail covers. And I say questionable because I have never seen anything like them. This doesn't mean rare, this just means that I have never collected stamps lol.

Definitely leave the stamps attached.

It isn't a rare cover but it does have value. How much value depends on the type of buyer. There is the value of the stamp itself. The machine slogan cancel is common but very collectable among postmark collectors. Some postmark collectors also look for special combinations of the slogan cancel and the circular date stamp (e.g. city, year, day of the year, machine used). Another venue is the postcard itself. I don't collect them but many people do. Its value depends on the picture, type of manufacture and the company who made it. Someone may also have a special attachment to the addressee or the content of the message. Who knows what's out there?
So there are a number of markets for this item. I'd suggest checking ebay sold listings in these areas to see what the actual value might be.
Dan

Check the date of cancellation on each cover and compare it to the first date of issue for each stamp. It's always possible you may have a letter mailed on the first day of issue that doesn't have the philatelic fancy cancel that says First Day of Issue etc, but if you have a stamp with a cancellation from the first day of issue it can add to its value significantly.