Petsmart stores will typically match the online price from their website. Print it out and take it in with you. I've done this in the past.
More recently, I keep an eye out for online deals with free shipping. I picked up a bunch of buckets of Reef Crystals from Dr. Fosters for ~$33 each earlier this year. The UPS guy found it rather amusing delivering all of them one day.
When the Big Al's local store opened up, they had a special price on IO - $29 a bucket. I stocked up on those when they were offered, too. Unfortunately, I've never seen that price again.