Why would that matter in the case of searching FedEx packages? Can’t we open up anything for any reason?
I've witnessed three types of police involvement. One is we have a package that we think is drugs and we call them and let them do their thing. I've never asked why we just don't open it, but I suspect that it's to prevent the claim that "The FedEx people opened the package and put drugs in it" or something along those lines. Joe Suspect could always allege that the FedEx employee was looking to ditch his own drugs for some reason, or who knows what. Better to let the cops go through their process.
The second is that the cops know of a specific shipment and they want to intercept it or control the delivery.
The third is the most common that I've experienced. Several cops come in during the sort and look at packages as they go down the belt. If one looks suspicious for whatever reason, they pull it and do their thing.
So to answer your question, yes, we can open 'em up but we don't in these cases because it's easier to let the cops do it.