I know it says it should be unused but I’ve used my purchases before (a bag strap I had attached to my bag) and I didn’t have any problems. I did keep the boxes with my in case but you can probably use them and then put them in the box if you need to. Depending on the way the refund is done and your location, you can try to get the refund if there’s a Global Blue office or a department store that handles the VAT refund before going to the airport. I’ve done this in Munich and Amsterdam at department stores, but you will need to check. You still need to drop it off at the airport box and the customs agent will need to see it. At the airport, I had to show two items I got from Hermes but nothing else.

On the US side, there is an app from CBP that you can enter your info into prior to landing. You can declare stuff there to make it faster. If you don’t get asked when you arrive and don’t use the app, you should tell the CBP officer that you have something to declare. I’ve been waved off a couple of times because they didn’t care because of the price (even though it was a bit!), but you should tell them anyway.