The LFS I owned (we just closed our doors for the final time today) gave no guaranatee on Discus either. Think about it, why would we? They have the ability to choose the fish so that isn't an issue. When I buy fish sight-unseen from a breeder they are guaranteed to be alive when I get them. So I guaranteed the same thing in the LFS, and we never ever sold any dead Discus. We also never had a Discus die in the bag on the way home to the customer's house. And after that point the risk is that of the customer's. If he/she can't land the fish properly, has unsuitable tank conditions, doesn't quarantine them then anything can happen.
Dave