The United States Postal Service are imposing a rate increase on First-Class Mail Parcels weighing between 1-3 ounces. These new rates will go into effect on the 28th of August 2016.
Currently a 1-3oz First Class Mail Parcel (whether posted with retail stamps or eBay labels) would cost $2.45. The price will increase to $2.62.
eBay say that they are negotiating with the USPS to offer the lowest rates possible, and will continue to provide discounted rates via eBay labels whenever possible.