@longtimejewelryseller,
Managed Payments has been in use for 5 years, and all eBay transactions have gone through MP since 2021. Many other sellers have been able to receive USPS insurance reimbursement for transactions that were processed by MP during that time, so it is not true that USPS will not accept MP documentation for proof of value.
There must have been something else about your insurance claim that led to it being denied. You have used a posting ID so that we can't see the actual listing, and you have not shared the actual purchase price of the item in question. That makes it impossible to review the situation to give you detailed advice. Sharing the item number, or screenshots of the order details page, would be helpful.
When other sellers have had issues getting an insurance claim approved, it sometimes turns out to be because the seller tried to claim the wrong amount in the insurance. The value should be the actual price that the buyer paid for just the item in the claim, not including any costs for shipping, tax, fees, etc.. Other sellers have reported issues because they tried to claim the full amount of the buyer's payment as the value, or the purchase price plus shipping charge, or the amount they had to refund, and received the same response message as you got.
If the claim for a lost package is approved, the USPS reimburses the item's value plus the cost paid for shipping, per Domestic Mail Manual section 609.5.4, so the cost of shipping is not included in the claim for value.
