A while ago, I tried the Fantastical trial, and when I used the purchase link, I was immediately prompted to a credit card payment screen. I paid and bought the software, and all was fine and well.
But later on, I learn that the software was also available in the MAS - something Fantastical didn't really bother explaining to me. When I contacted them for any kind of refund (to be used to purchase the app in MAS), I was told that they couldn't accommodate that since a lot of other users have requested the same thing - which already seems like a weird way of thinking.
They price the software in both places the same - meaning they earn more money, if people don't by it in the MAS, and I felt the entire experience to be very deceptive.
All in all, a really bad experience that I can't recommend any developer to imitate, as it does nothing to inspire customer loyalty. Especially when the developer has a financial incentive to not addressing the issue. At least from my limited perception as a non-Mac developer.
Again, this is the impression you get as a customer. I'm not saying this is the intent, but perception is reality in the real of customer experience.
Consider this another con to double-dipping, if you intend to do something similar.