Taco Bell offers the Free Doritos Locos Taco with any online order (or using the app) of $10 or more. Now the fine print:

"Offer valid on orders of $10 or larger. Limit 1 per order. Offer expires at 11:59PM PST on 5/31/2016. Available at participating locations for mobile or online orders only, while supplies last. Additional charges may apply for substitutions. Check your local Taco Bell store for availability. Offer redeemable once per day. May not be redeemed with other offers."