The scam isn't effective even if you do, since it will generate 1-star reviews and chargebacks no matter where the seller goes, so eventually they won't be able

The scam isn't effective even if you do, since it will generate 1-star reviews and chargebacks no matter where the seller goes, so eventually they won't be able to continue scamming more people. They'll be dropped by every marketplace and every payment processor. It's also a lot of work... most ecommerce platforms, marketplaces and shipping services support USPS's Automated Postal Verification system. As soon as the package hits a sort center and is weighed/scanned, the insufficient postage is noted and charged to the sender. If I ship to you through Shopify or Etsy or Pirateship or what have you with insufficient postage, the difference will be charged to my credit card before it reaches your post office, and you'll get your mail as usual. To have the insufficient postage reach the buyer, you have to use stamps or a guest account at the USPS website or something else that doesn't keep your credit card on file to cover the mistake. This isn't a widespread scam, it's self-resolving, and USPS is not actively enabling it.