In January my husband + I were on Crystal along with my brother & sister-in-law. We paid for their tickets, they don't have a Plat Amex, and they got the $550 also. So, not certain that the cruise-takers need to have a Plat card, I think as long as it's paid with a Plat Amex they'll be all set.

So, we got $1100 worth of on board credits just for paying with our Amex.