The 2 companies offering dog poop DNA collection kits both have the same idea and use the same basic principal.
Take a portion of the dog poop and put it in a preservative for shipping so the DNA does not degrade during the long shipping process.
Mr Dog Poop has a much larger container and asks for more of the poop as well as supplying a "Dog Poop Shaker" to thoroughly mix the preservative with the poop.
While Pooprints asks for just a nickel size portion of the poop and asks that you shake it up by hand.
Both seem to work, but mr Dog Poops kits and processing are only about 1/2 of the cost of the Pooprints kits.
And while we cannot speak to the results of the Pooprints method, from our experience the more poop we have, the greater the chance of making a match.
Leaving too much poop behind at the crime scene is just a mistake, because you cannot go back and test it if the collected sample does not yield viable DNA. So getting as much poop as possible increases the likelihood of a quality DNA profile.