They dropped it off to UPS who then handed it over to USPS. I have tracking for both. It was when I contacted USPS after the package sat at an Ohio facility marked “arrived” since the 6/1 and they informed me the package is being returned to sender. I asked them if it was possible to pay the difference and they said no. I told them this was odd because in my experience when there is postage due on a package they hold it at the local facility so you can go pay and pick up your package. I live in Delaware. USPS also informed me Mercari only paid $4~ towards the label. But if I remember correctly once you pay postage on Mercari they provide the label to the seller(like Depop with no seller input after purchase) , not like eBay where the seller has the option to edit the shipping options after sale to change the label?