Wills have different requirements determined by each state in order to be valid.
For example, only about 25 states accept handwritten wills as being valid, and some states require more witnesses to sign the will than in other states. I would caution you against using a kit purchased through the mail because it could easily be from some scam artist looking to make an easy buck. If you want to do it yourself, make sure it is a reputable firm supplying the kit and use it only for simple wills. Issues can be quite complicated, and you want to make sure your wishes hold up after you pass on. The cost doesn't have to be exorbitant.