1. https://www.kalzumeus.com/2017/09/09/identity-theft-credit-r...
2. This is my intuition talking here -- I am not a lawyer. If you have legal questions, talk to one. But, IMO it would be utterly bizarre if USPS record of when a specific piece of mail was received were not admissible as proof of claiming that company got your request.
PRETRIAL:
Sender (in a sworn pre-trial affidavit): I mailed this letter.
Addressee (ditto): I never got it.
Judge: We now have sworn evidence to controvert the Mailbox Rule (which establishes a presumption of delivery after mailing). That means I'm not allowed to rule summarily on that question. That in turn means we have to let the jury decide whether the letter was received.
To quote one of my former students: That's a conversation I don't want to have, because of what follows:
TRIAL:
Sender (under oath): I mailed the letter.
Addressee (ditto): I never got it.
JURY DELIBERATIONS:
Juror N (during deliberation): I believed Sender but I also believed Addressee.
Juror N+1 (ditto): I didn't believe Addressee.
Juror N+2 (ditto): I didn't believe Sender and I did believe Addressee.
VERDICT:
Now it's a roll of the dice as to what the jury concludes about whether the letter was actually received.
LESSON:
Use certified mail or other service that will provide independent confirmation of receipt, refusal, or reasonable efforts to deliver. (The Three R's of Notice.)