rabbut
I don't bite, all that often...
Well, with sensitive fish like Discus, I'd always recommend Drip Acclimatisation, basically with a air line. You start a syphon out of the air line, and use a gang valve to reduce the syphon flow to about 2 drops a second, and let this water feed into the bag with the fish in. Once the bag is full (or close to), you tip it out into a bucket, and continue filling up from the drip line until the fish have been acclimating for about 2 hours. Any pH change (or KH change more importantly) over this time will be slow enough not to affect the fish too much
How do you know the shops tanks pH BTW? Is it from testing the bag water of the last fish you got, or testing the tank directly?
All the best
Rabbut

All the best
Rabbut