The preset breaks two ways, and both cost money. A marketplace that rounds up to the nearest weight tier overcharges a seller shipping consistently under that line, and a marketplace that assumes a single default carrier never surfaces the orders where a competing carrier or service would have been cheaper. Neither failure shows up on the label screen; it shows up in the margin at the end of the month.
Packaging weight is the variable most sellers forget to control. A poly mailer, a small box, and a padded envelope do not weigh the same, and on a platform priced by weight tier, that difference alone can move an order into the next tier before the product inside it does. Weighing the packed parcel, not the item alone, is the habit that keeps a preset-priced platform honest.
Selling on more than one platform multiplies the problem instead of averaging it out, because each marketplace's preset is tuned to its own typical order, not to your actual catalog. A seller running the same product line across two or three platforms often finds the identical package priced differently depending on which marketplace generated the label.
The service question and the carrier question are not the same question. USPS Ground Advantage, USPS Priority Mail, and UPS Ground are the services that move most marketplace orders. On the marketplaces that price their label from a weight tier (Mercari and Depop among them), the service and the price are fixed by the tier you picked at listing time, not by what the parcel weighs or where it is headed. Poshmark goes further and attaches one flat-price Priority Mail label to every sale, with no tier to pick and no way to rate-shop or downgrade it. eBay and Etsy sell labels at a service you choose and price them by zone, but each starts from a default that most sellers leave alone. A platform that hands out a faster service by default is charging every seller for speed most buyers never asked for; one that hands out a flat rate regardless of destination is charging a local order the same as a cross-country one. Platforms that sell no label at all, WooCommerce, Big Cartel, and Discogs among them, leave the same choice to a plugin or an outside tool, and Shopify Shipping puts DHL Express on the menu next to USPS and UPS but still quotes whatever service the checkout assigned.
None of this changes what postage costs. USPS and UPS set their own published rates, and any shipping platform, marketplace-built-in or independent, buys against the same rate card. What changes is which service and which carrier get selected for a given package, and whether the seller sees that decision before paying for it. That is what each guide below works through: not a price war between platforms, but the specific weight break or ounce tier where the marketplace default stops being the cheap choice.
I'd Ship That prints discounted USPS and UPS labels, so a seller can buy the correct service directly instead of accepting the marketplace default. Ship Intelligence (Pro) applies the cheapest valid rate automatically once per-order comparison across multiple platforms stops being practical by hand.
Pick your platform below. For packing decisions specific to what you sell, use the item guides; for what a given weight costs, use the cost-by-weight guides.