This would be especially helpful for burnt drops which these private shoppers would be blind to. They may open the pack but they're not going to freakin dismantle shit or destroy whatever decoy is used.
And how is it even admissible evidence when some rando private employees are the original handlers of the evidence. I know once cops take evidence the chain is thoroughly documented
Why would fedex or UPS be "blind"? You are making a dangerous assumption that there is zero intelligence sharing.
On the separate issue, you are right that you would probably be okay for a single order using a private carrier. For example, between friends. But no vendor sending out lots of packs with a head on their shoulders would use a private carrier because of the warrant requirements for opening USPS mail .