$519.90 + $20.00 = $539.90
1. I don't know why, but, when it comes to heavier packages, the tape on the underside (almost always a single strip) tends to come loose or undone; in this case, it doesn't matter if the carrier is the United States Postal Service, Federal Express or United Parcel Service. I'd like you to - before you place the first box inside - please reinforce the underside of the second box (that is, the outer box) with more tape beyond the lone strip that holds and seals the bottom flaps. If you want to form a cross pattern, that's fine, but I wouldn't refuse a more thorough measure.
2. The last package I received was well-secured (and definitely a marked improvement over my earlier orders with NKG), but I've noticed that the corners don't always receive the best attention. Please ensure that the corners are sufficiently reinforced with ample tape.