Passing
reference in 2014 suggests that Endicia qualifies as shipping for American Express Business Rewards Gold. However, I see that Endicia changed ownership in 2015, and that often means a change in card processor and merchant classification. Still worth checking, Amex network merchant classifications are often different than MasterCard or VISA.
I can confirm that Endicia qualifies as shippign with AMEX Business Gold Rewards, but I am already maxing out the bonus category with that card.