Hi and welcome to the forum
If you don't have corals in the aquarium, you can raise the water temperature to 30C and it should kill off the white spot parasites. However, if you do have live corals in the tank, they won't tolerate temperatures above 28C so you can't use heat to kill the white spot if you have corals.
Triple Sulpha (Tri sulfa) used to treat white spot and was safe for marine tanks. I don't know if it still does. White spot might have developed a resistance to it over the years.
Praziquantel is a medication used to treat tapeworm in cats, dogs and fish and is meant to help treat white spot. I think the dose rate is 20mg praziquantel for 20 litres of water but check the instructions on the pack.
When we had white spot in coral tanks, we remove all the fish and put them in a separate tank to treat. Keep the fish out of the coral tank for 2 weeks and it should be safe to add the fish back in.
As for the fish in the bag, either return it to the store and ask them to hold it because your fish have white spot. Tell them you just noticed when you got home and don't want to put a healthy fish in a sick tank. Or set up a spare tank for the new fish.