Cancellation while in transit—any help?

So, some of you may be aware that USPS is notorious for just not scanning until it gets to a distribution hub. I shipped something on Friday the 13th and had a receipt, a literal photo of it on the counter with a time stamp and location via the photos app/map, and it hadn’t updated past “accepted by Wednesday the 18th AM.

Knowing they’re notoriously slow and this was a possibility I messaged the buyer to let them know even on Tuesday (Monday was presidents’ Day as well)and they had no issue. I’m a proactive seller so I just provided that documentation to Mercari just in case there was any issues given their tracking system only shows updates once it’s further along in the process versus when I drop it off., and randomly it was canceled three hours before it hit a distribution hub

I immediately filed an inquiry telling them it’s literally in transit has been in transit and I provided my receipt, etc., and it shows it’s going to be delivered to the buyer tomorrow. So of course they asked me for a receipt and proof of DROPOFF, which they already had and I gave them all of that and they didn’t respond for two days and the buyer was about to get something that was $400 for free and until I attempted to file a package interception Mercari didn’t respond at all.

The buyer said they’re no longer interested and that’s why they cancelled even though they trusted or was in transit. But they wanna buy something from me for $600??? Either way Mercari keeps asking for proof it was in their possession (USPS) prior to cancellation….which I’ve done several times….and they now said it says “return to sender” on tracking so it’s all sorted and I need to ship faster next time?? I called the buyers local post office and the said it’s still there’s not moving….mercari said if I have proof it was in USPS possession prior to cancellation I’m owed seller protection. Have done so 5 times. Do not know what else to do because it is NOT on its way back to me and is essentially lost and they won’t work with me.

Any tips??