Mercari review: Mercari selling/shipping platform — shipping label & DIM weight charge

I sold a "Disney Princess Hanging Art" item on Mercari for $82.80. Mercari's system generated a shipping label listing the package at 5 lbs with no dimensions entered, at a cost of $10.99. After the carrier (UPS) took possession of the package, Mercari retroactively charged a $27.92 shipping adjustment fee, claiming the actual package was 11 lbs and 22"x17"x7", reducing my payout from $63.53 to $35.61. I never manually entered the 5 lb/no-dimensions value myself — when I asked Mercari to identify exactly where that number came from, support (Charisse) only pointed to "item details entered during listing creation, including category defaults." Mercari refused to investigate the discrepancy further, split the fee, or explain why a package with such a large weight/size mismatch wasn't caught or flagged before UPS accepted it at drop-off, instead billing the full adjustment after delivery with no negotiation and directing me to a generic policy article. This leaves sellers fully exposed to after-the-fact carrier billing discrepancies with no recourse, verification, or transparency into how the original shipping estimate was calculated.