You can ask them to remove the low rating and chance are good they will if the buyer left feedback that is in direct conflict with the low rating.
Re: the post office, we've had more problems this year with them than ever before - not enough to get us near a 2% defect rating, but I am not happy at all with the trend of their service right now, either.
And, everyone else is spot on - buyers have to become expert eBay users and contortionists in order to AVOID opening a case when all they mostly want to do is ask a question.
Fix the things you can fix - you should not be switching packages, that's fixable with your in-house procedures. Use packing slips, match them to labels, and triple-check yourself throughout the shipping process. The fact that the buyers with the switched package both dinged you for not-as-described is pure bad luck - most buyers are pretty reasonable if you get the right stuff to them even after a mistake.
You can try going up the chain of command at the post office - don't stop with the supervisor, tell him you want to talk to the regional supervisor, and if he won't help you do that, call the post office main number. Will it help? No idea. I haven't -quite- got to the point of being frustrated enough to try it.
And, if you are looking for lower fees (comparable to what you are getting with TRS plus) and less stringent seller standards, you're not going to be happy at Amazon. They are more expensive even if you lose TRS plus; and if you screw up there, you are permanently done. Don't even consider it until and unless you've sorted out your postal and in-house mis-fulfillment issues, is my advice.
I've had TRS and TRS plus since each of them started. I have a high enough volume to absorb occasional mistakes by me or the post office or buyers who don't read. But it would not shock me to find I don't keep TRS plus all the time under the new defect system. It's amazing how quickly it can all go pear-shaped.