As Dorothy says, anything you transport becomes yours for the purposes of customs. There is no way to 'certify' anything or prevent customs agents examining it. You are responsible for any duties on anything you carry, and you will bear the penalties if they are found to be illegal. This includes the fact that if drugs are found on your person it is you who will serve the years in prison, no matter how often you say you were carrying them for someone else. I know personally several people this has happened to.
Because of this, you should only bring back for people things that you yourself bought in a retail store, or packages that you have checked for yourself contain no contraband. Even this last point is high risk, because drug dealers have a lot of expertize in hiding drugs inside seemingly normal things.
My advice would be to transport only things for people you have known and trusted for a very long time, and only things that are strictly theirs, not anything they are asking on behalf of someone else, or things other people are sending to them. The very fact that you are considering this as a possibility screams "Don't do it".