As a person who once worked for tips GIVE THEM CASH! As far as the spa tipping being optional only if your being cheap or didn't get good service which has never happened at any of Couples spas to us. Always great service worth a nice tip.
We stayed at CSA earlier this month and had the same FILA credit. I can tell you what everyone seems to be tipping because I saw the tip sheet! What I mean is, my masseuse was still down at the body wrap hut and I wanted to leave her tip at the desk. The attendant pulled out a tip sheet where she wrote down my tip, and I was able to see the other tips of the day. They were for either 5 dollars or 10 dollars, I noticed.
However, my husband and I tried to realize what a truly terrific treat we were getting -- a reflexology massage, deep tissues couples' massage and body wrap were all covered by our credit -- so we tried to tip accordingly. I believe we tipped $10 each time, except for masseuse who did the body wrap because we felt she was uniquely outstanding (we gave her $15).
Not that this is law or anything, but here is a general "tipping etiquette guide" article that someone posted on the old message board. It is not Couples specific, just general tipping suggestions. It covers much more than spa tipping too.