Comixology: 225 Free Scooby-Doo Comics
Scooby-Doo (1997-2010) [comixology.com]
Scooby-Doo, Where Are You? (2010-) [comixology.com] (last 2 comics are $2.99, so be careful)
Scooby-Doo Team-Up (2013-) [comixology.com]
You can press the “Add all to Cart” link to easily purchase them all. I believe there is a limit of 150 items allowed in your cart to checkout. Just verify that you do no have any pay items in cart when you finally checkout.