Saw this on my Facebook app, and posting on my phone, hence couldn't get a URL for it.
$20 off by using TUES20, order within the app. Valid today only until 9pm.
Facebook post says vouchers are limited.
EDIT: I just preorded for delivery tonight. My restaurant's minimum was $20 + $3 delivery fee. I ordered $20 of food and used voucher, and paid $3 total. Tried to stack vouchers, but doesn't appear you can in the app. I am an existing user.
EDIT2: Sounds like there is a $20 minimum cart (food + delivery fee) for the voucher to work. You also need to meet the restaurant minimum order rules.