Its like the post office getting envolopes by the truckload, then opening each envelope, read the content, to decide when to send the opened letter for delivery, either by foot or car, claiming that such a decision process will prevent envelopes from flooding the post office, coming into the post office for delivery in the last mile.
There is, however, at least one more dimension with postal or package delivery services. They offer different delivery priorities with different pricing, may have surcharges or refuse large content that the physical transport technically could carry, and offer sender-pays and receiver-pays options.
The starting hypothesis is based on envelopes of the same size, weight, colour, with no chemical powder in them, and all with the same stamp value... The emphasis, is the need to open the envelope to decide how to route them... F.