Hi Diane,
I just checked Costco again using your dates, and their rate is still $523 for those dates. That's $25 a day, all-in. If the rate you got is better, then stay with that. Make sure your Car Retals.com rate includes all taxes and fees, has free mileage, and includes a free extra driver. Most companies charge extra, and your final cost may be higher than they quote up front. Costco's rates are all-in. No surprises.
Also, be aware that rates do change from time to time. With Costco, you can book a car, then cancel and rebook if rates change. I often find a lower rate and rebook. I've saved plenty over the years doing this. There is no penalty to cancel.
As for the Costco membership, if you joined, had your rental, and then later found you weren't using the membership, you can cancel that membership at any time during the membership year, and they'll refund your entire membership fee. So you have nothing to lose on the deal. (On the other hand, you may choose to keep it - you may find a Costco warehouse 20 miles away from you may be a worthwhile thing, as their merchandise and such are considered better than other membership warehouses. They are definitely not all created equal.)
Good luck on your trip! (And I agree with Boca - your company rate isn't very good.

)
Dave