Substitutions, if Target ever does them, shouldn't ever result in the guest being charged more. To prevent shenanigans they should probably only be done by a TL, as in the TL has to log in to the cart and do it. It also really needs to be the same underlying product. So, the example of ordering one pancake mix and getting another shouldn't be allowed. Different brands have different recipes. What if the guest prefers the taste of the cheaper brand and doesn't even like the more expensive brand? We have no way of knowing that.
But, let's say a guest orders Tyson chicken breasts and we're out. We should be able to give them the Good & Gather chicken breasts if we have it because they're the same product essentially. But, we shouldn't be giving them chicken thighs obviously, or even organic non-gmo chicken breasts because that's not really the same thing.