Yes thats my only worry. UK customs not the USA ones that is.
I've included a copy of my invoice from Planet showing that it was brought from the Uk so hopefully that will help
it wont help unfortunately....as opposed to charging you for the marker, UK customs will charge you import tax on the lasering service itself in additional to any postage costs on top of that (only applicable to USPS express mail international, not USPS global priorty international). I was given this advice by HMRC customer services rep. Turned out she was right - I ended up having to fork over an extra £22 for my state-side laser job (from lasermygun.com) upon its return to the UK. Customs wont bother with your marker assuming you have the right documentation to prove it was a UK purchase but they WILL tax on on the lasering service because it was done internationally.
The reason being that USPS express mail is handled by Parcelforce at the international hub once in the UK, so customs are straight on your case at the airport. On the other hand, global priority international is handled by Royal Mail. These are processed alongside all the other regular international mail so it is far less likely to be flagged by customs at all.