While there is no “official” coupon policy on the CVS website, these rules have been communicated through emails with district managers and other corporate-level managers at CVS. Your store’s policy may differ.
CVS accepts manufacturer and competitor pharmacy coupons (i.e. Walgreens, but not Publix or Target).
One store coupon and one manufacturer coupon may be used per item. ECBs may be used in addition to these as CVS considers them more of a gift card, not a coupon.
During a BOGO sale, you may still use one store coupon and one manufacturer coupon may be used per item.
CVS does not permit overages. When the value of a coupon is higher than the price of an item, the cashier will adjust the coupon down to the price of the item.