If you mean 'Flat Rate' boxes, you'll be charged the Flat Rate price for every box you use. You can, however, use as many 'Priority' boxes as you need (to house your item), for one, overall Priority price.
For something like that and depending on the price, I would take the guitar to a UPS store and asked them what the charge would be to custom box and ship. They can custom box pretty much anything and they have the different materials to do so.
Just a thought. It wouldn't hurt to get a price anyway.
