Opinions vary ( see the epic thread Costco selling Hyatt Check Certificates (Not Applicable to Pre-paid Rates) - FlyerTalk Forums for more opinions ) . If it works, good.
If not, i kept the receipt, so can take them back to Costco for a full refund. Nothing ventured, nothing gained.
Edit: looks like I was both lucky, and a mere amateur http://www.flyertalk.com/forum/13977023-post1058.html . Someone understands the concept of arbitrage (http://en.wikipedia.org/wiki/Arbitrage)