I bought a uke from them recently, and it has the same problem. Buzzes on the C string (non-wound) when played open and fretted at the first and second fret. I don't have to play it very hard at all for the buzz to come, and it does come when strumming too, but obviously is more noticeable when plucking that individual string. Mine's not made out of wood (Clara), so it has nothing to do with humidity.
Reading your problems, I can only guess that the setup person at HMS has a very delicate touch when playing. I haven't contacted them yet, but I guess I should. I just don't want to send it back to Hawaii all the way from Europe. It will cost a bunch and take forever, and I really don't want to be without it right now. Think I might just take it to a local luthier instead.
/Max