I have bought food occasionally from Petflow.com. They occasionally have flash sales (10% off) for just a few hours, if you get their emails. Shipping is always free over $49 (and often free at lower amounts with a promotion). You can add in a few of their door-buster treats if you need small items to get over $49.
I've been pleased with them. (They will pester you to turn every order into a recurring, auto-ship order, so you have to make sure you decline all that, unless you want it.)