Shoppers is not a grocery store, so of course they're going to charge more for things like peanut butter, when compared to grocery stores like No Frills or Costco. You're paying extra for convenience, and they're selling lower volumes. A better comparison would be peanut butter at convenience stores.
This feels like unnecessary noise. Yes Shoppers Drug Mart, like corner stores, sell food at a large markup. They've always done this, they know that if you're picking up food there you're willing to pay a convenience markup.
Shifting any focus here gives Loblaws a potential out by just reducing prices at shoppers, which has nothing to do with what we actually want to happen.