Even if the seller does as promised, it is difficult for you to prove that it is invalid.
I need you to prove that you took it as required. How do you prove this?
There is also the need for formal proof of medical institutions at or above the county level. Will you pay for it?
In addition, if you get the certificate, the seller can veto it in one sentence and then declare it.
So don't take any nonsense about invalid refund seriously.