I don’t know if this will help anyone but I had to send some things back in May and June that I normally would send certified with a return receipt but I was strapped for cash so while at the post office I discussed it with them. They said it really isn’t as necessary any longer to get a return receipt because you can go online to www.usps.com and track it there and when it’s delivered you can print out that page for your records which shows the date and time of delivery to the address etc.
They told me they feel that it would be enough to satisfy the courts so I took them at their word lol and only sent my mail certified and saved myself over 2.00 per letter then I went on to their website and set it up to be notified when my letters were delivered(one of them was for a refund check but the other two involved credit and debt issues)and when I was notified and I printed those pages out.
It seemed to work fine. We didn’t discuss PO boxes tho, all items I sent EXCEPT for the refund were going to physical addresses. The refund check was going to a PO Box and I couldn’t detect anything different. I guess it would depend on if whoever you are dealing with would want to challenge the USPS with regards to their delivery kwim?