Welcome to the forums wamster007.
The chemicals you purchased from Costco are most likely Trichlor tablets and powdered Dichlor 'shock'. (I used the exact or very similar product from them for a little over one season as the cost during this months sale is very appealing.) Along with adding chlorine, both contain and will increase your CYA, potentially very quickly using the powder on a weekly basis, to the point of your pool being unmanageable in the event of an algae bloom. The higher your CYA, the higher you have to hold the Chlorine to get rid of the nasty stuff. Read up on the recommended Chlorine/CYA relationship at
http://www.troublefreepool.com/content/128-chlorine-cya-chart-slam-shock
Reading the pool school instructions and a few threads on algae and its prevention will lead you to the understanding that unless you are frequently draining (vacuuming to waste for example) and adding new water the Trichlor and Dichlor method will get you in a place (high CYA) where your pool is actually days away from going green because you might be targeting a certain static FC level that is no longer high enough to prevent an outbreak due to rising CYA.
I would suggest returning the stuff to Costco and buying the Clorox bleach they sell in three bottle packs at a very reasonable price. The Trichlor and Dichlor do have a very limited place/use in the methods used here. You could use the powder to slowly and carefully increase your CYA and you could use the tablets for short times when you might be away from the pool on vacation. But with the Costco bulk purchase you will have enough of both to last for several years of judicious occasional use.
If you make this move, be prepared to test your water for FC level and add bleach as necessary on a daily basis, along with the other testing done on weekly basis once things are dialed in. It is a commitment, but its one that puts you in control of your pool and will save you time and money in the end.
Good luck with the PH adjustments and keep coming back for anything else you may be interested in asking.