Civil issue. Police or immigration are just empty threats. The only thing they could do is take you to court which is not financially viable. You have nothing to worry about.
Normally when one is blacklisted due to an overstay the immigration officer has to go through a procedure to add the person to the blacklist and give him a paper w the notice of overstay and a ban of X amount of time. I would suggest trying a different land border before attempting using the airport.
Can you share what shipping company you used in the US and shipping cost?
I used V Pack once to clear customs and went back to them for a second time but the prices were a lot higher for clearing customs. I heard it is best to call ahead and negotiate.