Interesting. Any particular reason it should be USPS rather than an email?
Email is common, frequently ignored and is usually handled by automated customer care systems (or agents who are so poorly paid they have no reason to care. They may even have a hard time understanding English
USPS mail is more unusual (but takes a little more time and skill) and if sent to high ranking officials (think C suite) harder to ignore.
Most C suite occupants know English very well and have incentive to keep customers happy.
If all else fails contact one of the many media consumer helpers.
Bad publicity is one thing all C suite residents want to avoid. (You can even threaten to do it but make sure if you do threaten you are prepared to do it. Include your already written letter in the letter to the C suite
I have had very good luck with short well written mail sent to multiple officials with supporting documentation. Think CC, let all know who else saw it
There are many sites online that list officers of companies because of filing requirements for the stock market ( SEC.gov | Filings & Forms
is just one of them). With the right search terms anything can be found even secret USPS addresses.