In general, Costco does not accept manufacturer's coupons, according to its membership privileges and conditions page. Only coupons that are distributed by Costco are accepted.
Costco's membership privileges and conditions page outlines the acceptable forms of payment. It mentions, "Costco does not accept manufacturers' discount coupons or other retail establishment discount coupons (other than those distributed by Costco)." However, Costco does offer highly competitive prices on its products to save its customers money on their purchases. Paying for items at Costco requires cash, check, debit cards, Costco cards or American Express cards. Most other credit cards are not accepted, because additional credit card fees would prevent Costco from maintaining their competitive pricing structure.
The wholesale clubs may not want to deal with coupons because that is just another level of paperwork for someone to do and they have to pay that person. . Or manufacturers do not
The nice thing about Target (as opposed to Walmart or Walgreens) is that they have their own store coupons AND they accept manufacturer coupons. Let's say you have a Target coupon