How to confirm arbitration opt out receipt?

[gimmedaloot]

I sent two letters - one via regular mail and one via certified mail. The certified one was delivered weeks ago but Tesla is not picking it up from the PO Box. USPS is saying it will be returned this week if no one gets it. I presume the regular one made it just fine but I have no proof of this. I feel like I've exhausted my options at this point. Anyone face anything similar?