Hi Boots,
You might have to ask your guy to try a different post office, because funnily enough it seems some of the post office staff over there obviously dont have a clue about what they can and cant send.
Ok, now I just went and checked 2 separate backglasses I had sent from USA and the box was actually a bit smaller than your measurements ( 35 x 30 x 3 inches). It is possible that the thickness of the box may be pushing it over the cubic limits, so if you can reduce it to say 3 or 4 inches thick it may make a difference.
Both backglasses were shipped by USPS.
One came by EMS cost $106.95
The other one came by International Global Priority.$120
These are both options that USPS has so there is no way that it cant be sent.
Get your guy to ask the post office why its too big, they will have to have a reason and then you should be able to work around it.
Good luck