Settings → Shipping → Setup → Boxes → Add a USPS flat rate box: the Flat Rate Envelope (regular, legal, padded), the Small, Medium and Large Flat Rate Boxes, and the Priority Mail Express envelopes.
Which products use them
- On a product, Ships in: Packed automatically (your own boxes) or a flat rate box, with Up to N per box.
- For the whole store: Ship everything in… a flat rate box. A product's own choice wins.
No weight needed
A flat rate price comes from the box. We send the known weight, or 1 lb, which doesn't change a flat rate price. On the Ship card the package says no weight needed.
Mixed carts
Flat rate items go in their boxes first, and the rest are packed into your boxes. A carrier service shows at checkout only if it can price every package. USPS Priority Mail prices them all; other services price a flat rate item as a normal box only when it has a weight. If no service can price the cart, the shopper sees your fallback price.
Products with no weight
Products with no weight and no flat rate box use the store's default weight (Settings → Store). Settings and Products warn you, and on the Ship card the packer confirms the weight before buying the label. Checkout never stops.