I bought a used part from someone in the US only to have the package "returned to sender" for no apparent reason! Evidently US Customs is the culprit. I have copies of the shipping label here and it has no fewer than four official-looking stamps on it after it was accepted for mailing! Beware, this could happen to you! Interestingly. I have mailed stuff from here in Canada to the US without ever having had a problem.
Has anyone else been subjected to this lunacy?

I'm curious why you would think that an export requires US Customs approval? As far as I know, the only documentation required is for Canada Customs.

There are three players: USPS, Canada Post and Canada Customs.

It would seem (to an unknowledgeable outside observer) that the most likely cause was an issue with the documentation (likely incomplete) required for Canada Customs.

Hopefully you can get the issue sorted with the person you purchased the part from.

Hate to say it but we only use Fedex ground. It's relatively fast, but cost $25 for customs docs. It's the only way we can guarantee the customer gets it.

Customs is NOW a real pain in the ass. My last shipment that came Fedex from Germany was tied up in Customs for 10 days. I had bought 10 lenses, I had now become a automotive parts importer according to them. Had to submit NHTA paperwork declaring parts met all USA standards. FedEx said on a sidebar that politics were now in play

I sent some used parts to the UK and USPS said it would take 8-12 days. When it was late I started to check tracking. It sat in Chicago for over three weeks. It's been 1.5 months and it's still not delivered
USPS is run really well by Washington politics

The prime reason is the guy that runs the USPS is a jerk. That the USPS handles 137.1M first class items daily, with a reduced workforce and hundreds of distroyed high speed sorters, might be part of the problem.

While I agree that I too have experience with a decreased level of service from USPS, I don't think that is the cause of the OP's original question.

Yeah, when I was completing my rebuild earlier this year, parts deliveries via USPS could be up to 3-4 weeks delay (from a service that in the past was almost guaranteed 2-4 days domestically).

After several delays, I would tend to promote shipping via UPS or Fedex.

Unfortunately for the OP, the Canada/US Border is not fully open to non-commercial traffic.

I'm not sure how close the OP lives to the border. If close to the Lower Mainland (of Vancouver), it used to be common for "importers" to set up a PO Box in one of the close American Border towns (ie, Blaine, Sumas, etc) and pick up the parts directly and bring them across the border personally. Unfortunately, the US Border is not yet open for Canadians (non-commercial).

I noticed that all my shipments from Restoration Design (Ontario) over the last 18 months were all shipped from Buffalo for likely the same reasons. Customs documentation can be very tricky (both directions)...for the inexperienced shippers, there are bound to be issues that can hold up or prevent delivery.

Unfortunately, for the OP, there are very few options right now...however, as @Mikey914 has suggested, reputable shippers like Fedex/UPS which have experience with Customs documentation.

Be thankful that you don't live in PT Roberts Washington. Only way there is across the border that's closed. You have to enter and exit through Canada to get there (coming from or going to. Both ways.). Those citizens are fucked. Proper. They're worried about the supplies of life. Car part worries are secondary. At best. And Canadians that own properties there can't go to them.

Interesting, yes I missed being able to just drive across to Pt. Roberts to pick my 914 parts, new or used. But I have recently bought stuff from Pelican, PMB, TireRack, etc. and have them shipped direct to home without any problems. I’ve also bought used parts and had them shipped direct without issues. One thing I noticed is the shipper has to include a “commercial invoice” to show the value and type of contents, not sure if that was the problem, being missing or not filled out properly.

J.

I don't know either, but the tracking map clearly shows that the item in question went from the seller's local post office in NJ to US Customs in NYC. Then to somewhere in NH that I've never heard of and back to NJ! It never left the US, never mind getting as far as the Canadian border!! The seller informed me that it was rejected by US Customs and judging by the foregoing that was clearly the case!!!

I don't know either, but the tracking map clearly shows that the item in question went from the seller's local post office in NJ to US Customs in NYC. Then to somewhere in NH that I've never heard of and back to NJ! It never left the US, never mind getting as far as the Canadian border!! The seller informed me that it was rejected by US Customs and judging by the foregoing that was clearly the case!!!

Indeed! I live in Vancouver and have/had a US Mail drop in Blaine, WA which is now completely inaccessible because of the border closure! As a cautionary tale; I bought a stainless steel exhaust system from a seller in NJ last year which I asked to be shipped via USPS. The seller instead chose UPS - with whom I have had previous aggravation, but I digress - so that with the shipping cost and "brokerage fees" the cost considerably exceeded the purchase price!!