If you ship items that weigh 13 ounces or less and want to offer USPS First Class mail as a shipping option you will need to modify the Minimum Box Weight.
In admin under Store > Component > Settings > Shipping Settings, change Realtime: Minimum Box Weight to a small value such as 0.0625 (1 ounce).
Please note that USPS First Class mail has a weight limit of 13 ounces (0.8125).
