Agreed, clearly a "Top of the stack" piece containing all the due amount for multiple pieces in that day's mail delivery.
In 1951, the Business Reply Mail fee was an additional 1 cent beyond the regular postage of 3 cent a for a letter and 1 cent for a card. Without any calculation scribble, it is impossible to determine whether this was for 7 cards, 5 cards and 1 letter, 3 cards and 4 letters, etc., although there is a high likelihood it was all cards.
On a different tangent, it is somewhat unusual to find the due amount paid with a combination of postage stamps and due stamps. Here is an example with mixed franking from 1983, using a form to collect the due amount. Either way, the Post Office folks got their money, but perhaps in a different account.
