When the buyer has paid the seller in US dollars, the buyer will provide a digital proof of payment to the delivery firm who will then ship the goods to the buyer.
It is obviously not safe to provide the digital scan of a paper document, because such digital scan can be trivially photoshopped.
The problem can be solved with stablecoins such as Tether, USD Coin, TrueUSD, Paxos, Dai, or Gemini Dollar.
The contract between buyer, seller and delivery firm specifies that the amount must be transferred to the specific address for the contract.
From there on, the delivery firm can trivially verify in stablecoin's blockchain that the money is now present in this specific address. Hence, the stablecoin's blockchain will contain irrefutable, digitally-signed proof of payment.
Have you ever heard of any traditional bank which is capable of issuing an irrefutable digital proof of payment on behalf of the buyer?
By the way, would you even be able to send digital proof that you have paid last month's rent? Just think of it. Not one bank can email to you a digital document with which you can prove that you have made that payment. It is not that it would be a particularly hard thing to do. It is just that they are not capable of doing it.
