When creating one or more automation rules that apply based on weights, it's important to make sure that your rules account for all possible weights relevant to your business processes.
For example, if you want to apply USPS First Class Mail Package when the order weight is less than 16 oz, and USPS Priority Package or Flat when the order weight is equal to or greater than 16 oz, then you'll want to set up a pair of rules like this:
The criteria that is less than 16 oz...
The criteria is greater than or equal to 16 oz:
Notice that this pair of rules accounts for all values above 16 oz, below 16 oz, and including 16 oz. If you have any similar rules that hinge on certain weights or weight ranges, please make sure there aren't any gaps in those weights! Any gaps are missed opportunities for automation, so double-check that you're covering the full range of values that are important to your business.