Posting a painting from UK to US

My first post here - hello. I'm a beginner/amateur artist, doing mostly portraits in oil and acrylic. Hope this question is on the right board.  I'm based in London and doing a painting as a gift for some friends in Seattle. It's a 61cm x 76.2cm canvas on a stretcher weighing 1.5kg. I've never sent anything similar to the US and was quite alarmed when I did a google to see the customs requirements, though as the painting is a gift with minimal monetary value it appears it shouldn't be too problematic. Parcelforce's website has a list of restricted items which includes works of art. It sends me round in circles when trying to find concrete answers on exactly what I can send where via which service, but it seems I can use Global Express which will cost £72.70 when dropped off at a post office (I have allowed for packaging).  Parcelforce's site also can't say whether taxes will be due at the Seattle end or how much this might cost my friends. Does anyone here have any relevant experience which might help? Thanks :) Richard