https://shippingeasy.com/usps-2018-shipping-rate-changes-flat-rate-priority-first-class-w-tables/

Rates go up 5c for 1 - 4 oz. parcel, then for 5 oz., only 2c increase, then goes up again so that an 8 oz. package is a 41c increase, but then increase goes back to 2c increase for 9 oz. parcel, etc., etc.

Now I know why dogs love to bite the mailman!