If your pack is being held at the post office most of the time it's for a legit reason. Many such reason.. Needs a signature, your mailbox was full, you block your mailbox with car or trash can, something wrong with the address.
BUT, there have been accounts of LE waiting for people in the parking lot when they come out. This is all set up in advance with you telling them on the phone that you're coming in to get it. So just go in there and ask about it. If the clerk takes more than a couple mins to come back to the counter alarms should be going off in your head. And if anything else doesn't seem right you can always look at the pack and say you didn't order it and leave.
Also don't take your phone with you, stash that shit somewhere turned off