You wouldn’t think it would be difficult to find an insulated mail slot. When I had my front doors replaced many years ago, I had one installed for around $40 and bought a second one for my parent’s house.
Over the years, the front hinge had partially broken (damn kids) and the brass colour was looking dated. I had reversed the slot to put the broken piece on the inside of the house. Searching online, I did find one model but it was a whopping $47 USD plus shipping.
Checking big box stores, they only carry uninsulated metal mail slots that get super drafty and wet with condensation in the winter months.
I eventually found a replacement on ebay (search “upvc letter box”) for only $14 USD including shipping from the UK. The doors flip 180 degrees to minimize potential breakage from being overextended.