I can't tell what kind of harness that is. It looks like it has no breeching and doesn't look like a breast collar harness but also doesn't look like a neck collar either. I can’t find my glasses. I am assuming you are a beginner since you are buying your first harness, I would suggest getting a breast collar harness. They are easier to adjust as they fit a wider range of horse sizes per harness, it is easier to find someone to help with it and breast collars are usually less expensive.
Make sure it has a breeching; you will need this to keep the cart from running up on your horse from behind.