The clerk point-blank refused to try the Saks cards at all, mumbling something about "We don't get repaid (by Saks)" as though if the card goes through, Off 5th is somehow "left holding the (actuarial) bag" when all is said and done.

I got the sense the clerk had been trained (instructed) to refuse Saks cards, where escalating would've been pointless. I've ordered the item online successfully with those evil cards for in-store pickup of the very same item. It makes no sense that Off 5th online continues to accept this "liability" while training store staff to refuse Saks cards, unless they're unaware of such a loophole.