Who delivers for Purolator in the US?

I've got an item to ship from Canada to the US via Purolator. As I was about to buy the label, my buyer gave me a street address for delivery by UPS or FedEx, and a PO Box address if by USPS. I suspect Purolator hands off the package to the USPS for delivery in the US, but I can't confirm this anywhere on the Purolator site or the interweb.

A previous item I shipped by Purolator last summer to a buyer in the US arrived as expected. I thought If I ran that tracking number through the USPS site, it would confirm my suspicions, but the USPS didn't recognise the tracking number. Tracking on the Purolator site gives no clues.

Does anyone know this bit of trivia?

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Based on msau's suggestion, I went with the address for UPS for the label. When I dropped the package at the Purolator outlet, I asked. They confirmed it was UPS doing the final US delivery.