Our upcoming $15,000 Regent cruise booked through Costco Travel yielded us over $2,000 in rebates and Costco Cash, which was far and away better than any
travel agent quote. TAs were only offering use-it-or-lose-it shipboard credit which, on Regent, is mostly useless, since their fares are all-inclusive: all tours, alcohol, tips, business-class air, transfers, taxes, etc.
We'd be forced to use the credits on things we might not want to do (spa treatments, upgraded wines, certain tours) instead of using the money on reams of toilet paper and steroid-injected rotisserie chickens at Costco.