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Or is the fact that the majority of stamps from the UN are destined for collectors rather than postal use, make it a more profitable program?
UNPA is a very profitable program for the UN, because the actually delivery is done by the member nation where the UN post office is located (in this case, New York City).
Because of the large amount of UN postage available in the wholesale market, there were a number of people buying large amounts of UN postage at below face, and then using them for mailings and dropping them off at UNPA to be delivered by USPS. This went on for a few decades. A few years back, USPS and UNPA made an agreement to limit the maximum number of pieces of mail someone could drop off in one visit, thereby making it uneconomical for private large volume mailers. That pretty much cratered the wholesale market for mint UN stamps.