Typically they want confirmation of payment guarantee direct from the horses mouth and not some letter that could have been typed up by somchai the taxi drivers daughter.
FWiw, I had a mild stroke 10 years ago, presented to admissions at a large private hospital in Pattaya and the wheeled to ER.
I laid in an ER gurney for 7 hours with no treatment or assistance whilst the insurance company was deciding iff they would cover the bill or not.