Personally, if there were more affordable shipping options on light-weight packages...I would definitely dump the USPS.
You are very lucky that you haven't had any issues with tracking. You must have a great postal office near you.
My local office refuse to scan in packages unless they "aren't busy" (very small office with very little traffic)....making the tracking info quite misleading. I've asked many times about this, they said they system "auto-updates" some of the tracking info. They also delay many of my packages by 2-4 days, if I drop them in the blue outside mailing stations throughout town. The postmaster's response..."did you purchase insurance?". Clearly....I did not purchase insurance on a $10 item.
As for the UK package: I have shipped 3 packages to the UK in the past 2 months. 2 of them shipped using first class USPS postage, and the other was a USPS small flat rate box. 1 of the first class packages had every step of the process scanned in with accurate dates. The other first class package, has missing steps and does not say it has been delivered, even though it was. The small flat rate package (which was estimated to be delivered within 6-10 days, by USPS.com) is missing tracking steps, has wrong tracking steps, it has been 15 days....and the package was JUST scanned into the UK today! My flat rate client is very upset being as the package was time sensitive. (The tracking info claims this package didn't move from my local office for 6 days, then traveled from FL to NY, back to FL, back to NY, and is now in the UK)
Way to go USPS!! (as sarcastic as possible)
My advise on handling a missing package; although it does not happen frequently...it does happen. Between Ebay and Etsy, I've had it happen maybe 3 times out of 300+ packages a year. I typically 1st respond with:
"I'm sorry your package has not arrived yet. The USPS tracking info says it was delivered as of ______ at ___:____. Would you mind verifying the shipping address, just to make sure it was sent to the right place"
After they have verified the address (in some cases they have given me the wrong unit or building number), I ask them if they live in an area where a neighbor could have accidently gotten the package. I've also used the "I'll make sure to file a report if we can't find the package". With these approaches, only 2 of my packages out of 1000+ have actually been "lost" (and one of them was because it was stuck at customs...which I've learned from; to use very basic descriptions on customs forms now)
After trying to "find" the package without success; I would offer a refund...or a re-ship at their risk. If the second package does not make it...it's on them because they could have accepted the refund.
Good luck!