Yes and No (how is that for a sitting on the fence answer).
First, you can no longer assume letter mail takes 3-4 days from the US. Second, assume that payment will not be sent for at least 2-days. Give the guy a chance to get to a post office to purchase a money order, stamps and mail it.
However after 12 days from end of auction, there is nothing wrong with sending out an unpaid item reminder although I tend to send out a standard email follow-up and not eBay's automated notice which can be misinterpretted as a threat.
Remember, when a Buyer has a problem with a Seller, he always threatens that he will report the Seller to eBay falsley assuming that any notices to eBay can cause suspension or other penalties.
That being the case, by receiving an eBay notice he has likely assumed that you reported him to eBay despite his communicating with you that he is getting payment out to you.
I also would not tell him that he has been blocked because if he hasnt mailed payment, he likely wont now.
How will you feel should his payment arrive postmarked only 5 days after the auction ended and it was simply tied up in the system for a couple of weeks?
Similarly, when payment arrives and you ship the item out, how will you respond to him should the parcel takes as much or more time to reach him?
Again, nothing wrong with notifying a customer about their payment not being received however given all the threads addressing shipping delays, I wouldnt be so fast to use eBay's Unpaid Item Notice.
I suppose it is the same thing looking at how I would like a customer to advise me the item they expected didnt arrive yet.
Would I prefer them going through eBay or simply having them send me an email stating the item has not yet arrived and allowing me the opportunity to try to explain or determine why it hasnt arrived yet? Easy,I would prefer a direct email from the customer.
I guess in your case, its not the message, its the messanger.
Malcolm