They say good things come in small packages... but when you use Pirate Ship, USPS® and UPS® do have a minimum package size requirement!
Minimum package dimensions for USPS and UPS labels on Pirate Ship
Minimum Box Size: When shipping a box, the minimum dimensions are 6x3x0.25".
If your box is smaller than that, USPS and UPS could technically reject your package... although we rarely see this happen.
Put Small Boxes in Poly Mailers: It's best to use a box that's at least as big as the minimum dimensions... but if there's no way for you to do that, we recommend putting the box into a small poly mailer envelope when shipping it.
Learn more about placing boxes inside poly mailers, also known as the "Box in a Bag" method.