Use code 20OFF at checkout. Online orders only. Today only. I don't know if this is localized.
At the Delivery or Pickup screen there is a box to enter discount codes on the lower left side. If you have any DP Hearts you should first add the promo code and then choose to keep shopping. This way the code applies to the base price before the discounts of the Hearts. In other words 20% off of $7.45 is $1.49 or $5.96, but if you use Hearts and add a free drink worth $3 then the 20OFF code will apply to $10.45 so you get a discount of $2.09 applied to your $7.45 total making your grand total $5.36.