Same here but I also live in the same metro area as this guy. The prime packages being shipped via USPS seemed to start once the distribution warehouse opened up south of Minneapolis.

I noticed most if not all of my packages delivered via traditional prime were via USPS if it was directly through Amazon. A few recently were through Amazon couriers (suspected this was to aid extra volume of the holiday season). If the item is Prime eligible from a 3rd party seller and/or fulfilled by Amazon item those would come via UPS if the package was bigger than a mailbox could fit (anything smaller they'd sometimes come via USPS).

All my subscribe & save items will go via UPS though. But, I don't think I've ever had a Amazon package delivered via FedEx though (or if it was it's been a long time and don't recall).