I just came back home from Costco. I found they have a whole shelf of coupons for Costco.com purchase. I have tested one and it works. Each one is good for $25 off $250 purchase on Costco.com and have different 7digit code on them and expires on 12/31/2016. You may save another 5% by using Chase Freedom or other cards that offer you additional cashback.
Do not PM me to ask for code. I leave the coupons there for other people to pick up and only get a couple for myself.
Below are the fine prints:
Only valid on Costco.com. Not valid in warehouse. One coupon code per Costco member. Ineligible purchases include: new Costco memberships, Costco membership renewal, gift of membership, tires, online gift certificates, Costco Cash cards, custom installation services and online-digital printe orders. Not valid for orders at the online Pharmacy, Costco Travel or Costco Business Delivery. THe merchandise total in your cart must equal $250 or ore before tax and shipping to receive the $25 off. The coupon code must be entered in the promo code box at checkout to be valid. This offer may not be combined with any other offer or promotion. Offer expires 12/31/2016.