no you have to buy it in the box...

I actually did the math and found out that it would be cheaper to buy the underlayment from Costco since you get 150 sqft for 30 bucks and at Home Depot you only get 100 sqft for 25 bucks.  Plus, you'll get all of those great spacers that you can throw away!

That is not true, I talked to a Costco manager and they informed me that I could bring it back even if I did not use Harmonics underlayment.  He also told me that if I layed it out and installed it and I didnt like it, rip it up and bring it back for a refund.  Cant get a better warranty than that.  Where did you get your information?  I am glad I do not shop at your Costco.  

My friends dad bought a gas grill from Costco last spring and used it all summer and then took it back and said that he didnt like how it didnt cook evenly and they refunded his money on the spot.  Costco has always been really good about this type of thing.  But, it might depend on what store you are in and what manager you are talking to.  Good luck.

I am going to put down the Harmonics laminate flooring in my basement. I
have already laid down a product called Subflor, which provides a
moisture barrier and OSB underlayment. Therefore I do not think that I
need the underlayment provided in the installation kit. I expect it to
sound like walking on a wooden floor. However, I will buy one kit for the spacers, etc.

A note on the quality of products at Costco: I know of one example where
Costco has sold low quality items; Gargoyles sunglasses. These
sunglasses were hot items at one time, and were sold through a major
chain of stores. But quality went south at the Gargoyles mfrg plant, and
the major chain rejected an entire lot of sunglasses because of a large
number of returns. This lot of sunglasses ended up at Costco. Sure,
Costco has a generous return policy, but they are playing a numbers
game. Predicting returns and pricing to cover the returns is a science,
and they are very good at it. So sometimes the items at Costco are not
the highest quality, regardless of the brand name, warranty, etc.