Have you already bought this stuff? If not, don't. To raise alk, all you need is plain baking soda. I'm sure it will be way cheaper than BUFFER. I don't think the ph is going to drop from 9.0 down to mid 7s on its own. Add the muriatic acid as outlined above.

Your free chlorine is low. What you should do is test your water yourself. Get a kit and then you won't have to rely on a pool store. You should take 2-5 minutes each evening and test your ph and chlorine. Add enough bleach each evening to raise your FC back up to 5 or 6. That should keep you from dropping below 3 by the next evening when you test and add bleach again. Use just bleach instead of the pucks. The pucks will add cya to your pool. Your cya level is already at 40 which is ideal. Many pool problems are caused by these pucks. Continuous use of them will push your cya so high that it will cause you to have to keep your chlorine levels higher and higher or else your pool will turn green with algae. If it were my pool, I would not use the pucks. Save them for the few days when you might be out of town and not be around to add the bleach in the evenings. At this point, since your CC reading is 0, you do not need to shock your pool. If you maintain adequate chlorine levels, you really very seldom should have to shock.

By the way, how does your water look? Any more questions, ask away. Ther are lots of people here who can help you.