USA customs problems?

No, but don't send that much stuff to the US, only occasionally. Put in your terms and conditions for future "you are not responsible for the activities of customs or their charges" as I've had someone who claimed I had poor customer service as I wouldn't pay their customs charges. Unfortunately, with all customs departments there is no rushing them and you have to wait your turn. Your customer is being unreasonable and all you can do is point them to the US customs department.

Very poor re 16 days, although I've had one sit for 4 weeks in UK customs and then they decided to open it to find it was full of what we said which was stencils and face paint. They then wanted to see all paperwork regarding the item. Stupid thing is I had another shipment from the same company come through while they were still holding this box and it sailed through no questions asked.

Sure these customs departments stick a pin in a list to decide who they are going to irritate today.

Unfortunately, as I've discovered over the last 10 years there is no arguing, pleading or anything with them. You have to go with the flow. Its out of your hands once they have the parcel and you have to wait it to run its course it would seem.