Pool store testing is very hit or miss, sometimes they are good but more often than not they can be way off. This is one of the big advantages of having your own good quality test kit. After only a couple of tries your results will be more accurate than the pool stores.
Try putting the CYA into a sock and put that in the skimmer. The water flow will help it dissolve and the sock will keep it off the vinyl, where it could cause minor damage if it sat in one place for too long.
Try putting the CYA into a sock and put that in the skimmer. The water flow will help it dissolve and the sock will keep it off the vinyl, where it could cause minor damage if it sat in one place for too long.