When you think about it, a $2 lettuce doesn't really cost $2. It costs a lot more than that. If you calculate in the environmental cost of the transport that brings food hundreds or thousands of kilometres, add to that the damage done by pesticides, herbicides and artificial fertilisers and top that off with eroded soils, the excess water used to grow that lettuce and the nutrient runoff into waterways, and you start to get an understanding of the true cost of that lettuce, and food in general. It's not just the item, it's the system of production and transport that needs to be calculated in.
When you go shopping, take a basket or cloth tote bags with you so you're not relying on plastic bags to bring your food home . Make some small net bags for bagging up smaller items like tomatoes, potatoes, apples and carrots so, again, you don't have to rely on plastic. Don't buy things that have a lot of packaging, and make sure the packaging you do buy, is suitable for recycling.
Further reading about shopping locally:
UK and here
Using net allows the storekeeper to see what's in the bag without opening it - they like that, but you can use any strong and lightweight fabric.