WoW! Just interject with a little law enforcement info, cuz my put me in contact with police officers regularly:
1) In the U.S. there is such thing as personal use for any "controlled substance"! For example if you are caught with 6 cases of D-Bol they will hit you with a "Possession with the intent to distribute" charge. Which will always carry heftier penalties.
2) YES! You can be "arrested" even if you don't sign for a package. Arresting someone doesn't necessarily implicate guilt, just "reasonable suspicion". Can you imagine the feds simply walking away, if a terrorist orders truck load of anthrax but smells the feds and decides not to sign for it?
3) This whole "flagging" thing is true to an extent due to the fact most letters in the U.S. are directed to their destination via computers(saw it on discovery channel). My only issue with this is the amount of man-power required to chase down everyone who mails something that they shouldn't. Everything from aerosol cans to Hydrogen peroxide is prohibited from the mail service. I really dont see them chasing down some guy with $100 cycle of d-bol when they have people trying to ship meth into the country.
4) If they do bust you with out signing then assume they have you "dead to rights" on some kind of transporting a controlled substance over state lines/internationally. So NEVER, EVER, EVER, EVER, talk to them, immediately lawyer up. But the likely hood of this is slim because it means they really had to do a lot of foot work i.e. ran your credit card, staked out your house, or otherwise been investigating you(which is unlikely unless you are a dealer or they started some new "get tough" intiative)
5) The whole busting your "girlfriend" thing for signing is kinda true and kinda not. Because the police have to prove she is the one that ordered the stuff. They can arrest her and hold her for questioning, but it would be hard for a DA to prosecute her. Now if she rats YOU out, now you are screwed!