Phone 1-888-211-5977 and advocate to Costco that you want it continued in the future (I did).
This was apparently an initial trial run and it ends Sept 3rd (today).
Surveys from those who purchased (I am one) have to be completed in order to get the cash card.
Results will obviously determine whether the program continues.
However ...
There should be a GM incentive program in the coming month or two for model year end selling that will help.
Just speculating on that though
New C7 Corvettes will likely have discounted pricing continuing (especially where I am) as Dealers want to gain C8 allocations for future releases.