Slightly Meta: Travel tips for people traveling internationally with vintage equipment?

I'm actively working on show planning for VCF PNW and I'm noticing that we have a few international travelers planning to attend and exhibit their machines/projects. I'd like to put together a FAQ for the logistics of traveling with vintage equipment across the US border. If you have ideas please let me know. For example: Should I plan on providing letters in advance stating that a person is a registered exhibitor at our show, including details like the show location, dates, times, and contact information in case there is a question about why somebody is carrying strange looking equipment into the US? Is there any sort of paperwork or customs form needed even if nothing is being sold or left in the US? Any other gotchas to look for? Thanks, Mike