A recent thread had an example of a Business Reply Envelope with several postage due stamps attached to pay the postage and fee.
I didn't want to high-jack that original thread, so will start a new one to illustrate that when the volume of Business Reply Mail (BRM) is large, the stamps are often attached on a separate post office form. The following three forms are for the largest amount of postage due paid for BRM on a single day that I have ever come across: $1499.52
There was originally four forms involved, the first one with the $3.52 , and three follow sheets: two with a full sheet of 100 of the $5.00 postage due stamps, and then the fourth with 99 of the $5.00 stamp and one $1.00.
When I acquired these from a dealer 30 years ago, he had a stack about two inches tall of these forms. I must have searched through them a dozen times looking for the missing sheet of the $5.00 stamps to no avail. Someone else beat me to it, probably just wanting a used sheet of the stamps. So all I have are the three shown here.
This is the first sheet of the set. Note it states at the bottom that three sheets will follow this postage due bill form.

This is the second sheet for $500.00

I don't have the third sheet, which was for $500 also, but here is the fourth sheet for $496.00
