Then the delivery guy is not allowed to delivery the package.

Sorry, I should have made it clearer that this system works best with integration with a delivery company. And that company would need to have a smart phone (for the delivery guy to have) or such like to see that the QR code has been scanned and verified.

But here is a way it would work without the integration:

The buyer is expected to scan the QR code on delivery.
The UPS guy would no nothing about this, and not care
The buyer should scan when they have taken the package indoors - ASAP

If they do scan - no problem.

If they are trying to scam:

The Bitcoins are held in escrow
The seller has proof of delivery (via UPS)
The seller has proof that the delivery address was linked to the buyer's public key
The seller at least has the Bitcoins in escrow

At this point, if the issue is unresolved, the dispute is passed-over to a human to pass judgement etc.
Human has 3rd key to release Bitcoins with the agreement of any one of the parties in dispute.
Human cannot access the funds themselves.
They can view the UPS proof of delivery etc.
Plus they can call the buyer on their mobile/email and discuss etc. If no reply or daft excuse - it's a scam, release the Bitcoins to the seller
If UPS are at fault the seller can start a dispute with them and claim from UPS - then the Bitcoins are released back to the buyer.

I think this would work very well and is easy to implement. Surely it's going to deter any potential scammers big time?