I doubt any of the delay AT KEH is because of the USPS failure to pick the stuff up. USPS will even pick up a package at a residence if you call them. If KEH either doesn't have the package ready to ship or it just sits on the table that's not the fault of the PO.
I've shipped to Europe, Canada & Asia using the PO with no problems. Either using Priority Mail or Registered.
I Used to work for the PO & they do have their problems but dollar for dollar I believe you come out ahead.