Shipping costs are expensive so reducing these costs can have a huge effect on the bottom line. When designing packaging it is important to keep shipments as light and as small as possible. For smaller shipments being sent to residences the USPS has special rates for shipments under 1 lbs that are significantly lower than any other rates.
Shipments under 1lbs qualify for USPS first class rates. First class rates are just dependent on weight and incrementally decrease every ounce. The rate is fixed for shipments for all domestic shipment regardless of the origin or destination of the shipment. First class rates also take 1-3 days for delivery.
First class shipments are a great deal but as soon as your shipment goes over 15.99 ounces your shipment is not eligible for these rates. If you can design your packaging and fulfillment in such a way that you can keep shipments under 1 lbs it will save lots of money in the long run.