I, too, have no home delivery so all mail is picked up at the post office. Let me say that my DH and I shop on-line a fair bit and I'm often surprised when picking up my mail. I've had exactly that thing happen to me, except we get one card per package. I had handed over my 3 cards and received only 2 parcels. When I questioned it, the postmaster said two cards were for the same parcel. Seemed odd, but I couldn't check. A few days later, there was another parcel card in my mailbox. This time a different postal worker, who confessed that they had misplaced one of my parcels. AHA!
I LOVE picking up the mail and getting a parcel :D
Hopefully they find your missing parcel.