As long as the package/drone size is kept small and there are appropriate safety precautions taken it should be as safe if not safer than current delivery methods. Simply designate a operating zone for them (say 300-400' AMSL), allowable weather conditions, require any operator (or group of operators) map all "no go" zones (airports, substations, wind turbines, etc) in the drones operating area before the first drone leaves the ground, require all of them to have a interoperable transponder broadcasting location, speed & direction, etc, and make the company who sends out the drone responsible for any issues it may cause (via some kind of insurance probably). Sure once in a while one of them will malfunction, but so do normal delivery trucks. Just Google Image "ups truck crash" and you'll see some very poignant examples.