Note the "in transit to destination" is a fake event automatically added 24 hours after no scans. Assuming it wasn't stolen, it's still in NJ. Sitting in a pile for 10+ days for no apparent reason despite being priority mail.
21 days seems pretty excessive to be stuck in customs. I've sent a fair bit of stuff between UK and US and I've never had it take so long to clear customs. Plus they usually explicitly say that it's cleared customs.
It seems very odd that it would take that long.
Did you pay the right duties and tax on this? It may be that it's still with customs and they have absolutely no idea what a giant box of old SNES games is worth?
Sometimes. This can also happen before customs so that the company does not have to deal with it. It's amazing the abuse that other carriers do to USPS because they have a constitutional responsibility.