You could also get some lanscaping stakes and stake down the fence between the posts - if they are 10 feet apart you will need to do this 2-3 places between the posts. In addition, the least expensive fix would be to lay a 1-2 feet of chicken wire or harware cloth flat on the ground along the fence, attached to the fence and staked down along the edges. If you apply it flush to the ground you can mow over it.