WW Buck and slugs at WallyWorld run around $6 and some change for 15 rounds, so assuming you are happy with the S&B Buck it sounds like you got a decent deal.
Barrel loength is measured from the breech / bolt face. The situation you have encountered is common, but you should be fine so long as the barrel measures 18 1/2" from the face of the closed bolt. Realize, however, that the average LEO is not going to be familar with the correct method of measuring...probably not with the correct minimum length(s) either. In the somewhat unlikely event one should accuse you of having an illegal shotgun, I would advise you to be polite and advise him of the correct method of measuring. (This is the method used by the BATFE) If that does not work, make sure you get a Property Receipt from him / her for your shotgun. It will eventually be returned to you if you ask for it back. (Aggravating, I know, but better than a night in jail IMO)