The Post Office has a monopoly on everyday stamped mail, that's why stamps have increased by 400% while US Mail small packages have barely went up. The USPS is subsidizing their small package biz with stamp money that FedEx and UPS can't engage in to compete with them.
To open every package or even 10% of the packages FedEx and UPS ships would be a big fail and would basically end any type of efficient process. Packages delivery is almost entirely automated, there is no way a driver could open, dig through and search and then repack a box in any way, it's impossible. The cost would skyrocket and would put this type of shipping out of reach, when that happens all you have left is that good old Post Office Mail - no thanks.
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