It really depends on the store owner's policy. In your agreement what happens if fish die when they arrive at the store, are they responsible or is the fault at you.
If they are responsible then let them quarrantine and check the fish for disease, malnutrition, etc. If you are then pick them up right away, before they even un-bag them. Place them in a spare aquarium and treat them. Run a full course of treatment for internal and external parasites and ensure they are in good health before adding them to your setup.