Unfortunately, things like flour and canned tomatoes are heavy to begin with and are going to eat up shipping charges whether you like it or not. Where the shipping charges becomes really burdensome is for perishable items, like fresh cheeses, that have to be shipped one- or two-day air. But if you plan to buy a lot of different items at one time, I would still look at places like PennMac because you may find that your total shipping charges are less than if you cherry pick places for individual items and pay several shipping charges. So, I would make out a list of what you want and do some comparison shopping, including shipping charges.
Peter