Postal Or Revenue Use ?

Saw a lot of 4 covers on ebay with basically nobody bidding on them. I saw this cover and figured I'd toss a few bucks at it and actually won the lot for $3. This is the one that caught my attention


Improper use of the postage stamps ? I can't think of a domestic rate of 6¢ during this time frame unless it was way over weight. It appears to be some type of payment ?







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Postal. The rate at the time is 3 cents per half ounce, so this is a double rate cover, further evidenced by the wrinkles along the bottom from the wad of contents. It has a manuscript town mark across the bottom and pen strokes to cancel the stamps. Somewhat unusual as it has a notation of the contents written on the outside BEFORE mailing, rather than the usual docketing added afterwards by the resipient.

I agree postage, not a revenue use.