I hate to tell you but I don't think they are scamming you. When you have an algae bloom you need to shock and test your water two or three times a day, not just once. FC levels can drop that fast.
We always recommend you test yourself with a FAS-DPD chlorine test. I cannot remember off the top of my head if a CYA/stabilizer level of 20 or 25 is required (about 2.5 gallons of the LC you are using)

You also want the store to test for copper...