I am really sorry to hear of all your troubles. I have been bringing shotguns into the USA for sale at shows for some years now and only run foul of customs once at JFK. A 1st rate customs broker (no longer with us - but his son is) got me out of trouble in 24 hours on a Jewish holiday! I learnt my lesson and play it strictly by the book, no nods, winks or funny stuff and my stock always glides straight through.
More up to date, I can thoroughly recommend Hipage at RDU in NC and Montgomery at PHL. They both know their stuff and their prices are entirely reasonable.
They have both dealt with pre- and post- 1899 guns for me.
And no, I am not on a retainer!