AMS has paid showers available in the non-Schengen area, at the airside Mercure hotel. €15 or something like that. There is also supposed to be some kind of functional free shower on the upper level (where KL and other airline lounges are) but I don't recall using the existing ones and they require your own towel and toiletries as far as I've heard -- I certainly wouldn't rely on this, but if it's not available/functioning and/or you don't want to have to deal with your own wet towel or using paper towels to dry yourself, then the Mercure pay-for-use airside showers and/or the lounge showers are the better options.

The $85 for 60 day WC temporary membership is a good deal but I am not familiar with whether or not you get a card in time for presenting to KL lounge staff at AMS. Also, the non-Schengen KL lounge showers can often have a waiting list -- a long one sometimes -- while the pay-for-use airside Mercure shower-toilet room at AMS has more than plenty of facilities to eliminate most all waiting time/list concerns.