Does the buyer or the seller pay Vinted shipping in the US?
The buyer. Vinted's shipping help page reads that the delivery fee is paid by the buyer at checkout, and Vinted then sends the seller the corresponding prepaid label to print and drop off.
Does the seller ever pay anything toward shipping?
Only when the seller offers a Bundle discount that absorbs shipping, or agrees to cover the cost as part of a Make an Offer negotiation. Neither is imposed by Vinted, and the platform's seller fee is $0 on US listings, so the shipping revenue path is fully routed through the buyer.
What if the package is heavier than the buyer's chosen tier?
Vinted quotes the fee off the seller's declared weight when the listing was created. Underweighting the listing is what causes label upgrades to fall on the seller after a sale; declaring the true packed weight up front puts the correct fee in front of the buyer at checkout.
Who pays for a return on Vinted?
Vinted's Buyer Protection funds the return label when a claim is opened for an item not matching the listing. When both parties agree to a return outside that flow, they arrange and split the postage between themselves, because Vinted's help pages do not fund a private return outside a dispute.