I have used this benefit for years and I have always used it through my TA. Look for a travel agency that says it provides AMEX benefits. I have used other AMEX cards with no issue so far. I only did that because of doubling up with AMEX cruise offers. Now that you can only get one offer for all cards, I may change. However, the AMEX offer for $ and the offer for points are different and both can be used. If your cruise is far enough away, you can use it multiple times and you can overpay and the balance can be used for shipboard credit. Or at least it has worked that way for me on HAL.