I'm wondering if using a discount coupon that is from National website (such as $20 off weekly) voids the credit card CDW coverage?

I read various threads on the forum and it seems American Express is very restrict but Mastercard/Visa are not. I called Capital One Mastercard and received conflicting information, some agents say it doesn't void the CDW coverage while the others say it does.

Does anyone have any reliable information or personal experience?

I've been renting from National for years as I don't own a car and always used these coupons without any concern, but recently I read some horror stories that the renter ended paying $$$ because their credit cards declined their claims due to using coupons.