Costco squeezes the margins of their supplier more than BH. They’d only go with them if they have toA lot of consumers (especially from the EOS RP target market) don't look at B&H, but visit Costco every week or two.
Costco means volumes. If Canon (unlike Sony) can deliver the volumes that are meaningful to Costco, Costco is a very good channel to promote EOS R brand.
But of course, Canon has nothing to sell there (yet) until RF 24-240 is ready.