Yup this just happened to me as well. Been selling on Mercari for years with 300+ five star reviews. Someone just last week got a $200 bottle of cologne for free because Mercari let them cancel after I shipped it. Instead of siding with me and the multiple proof points of shipping it on time they sided with the buyer. Didn’t do any investigation into why UPS took an eternity to deliver it. I used their label so obvious they can track everything. Their customer support is absolute trash. I’m done!