Speaking from personal experience the problem with UPS is that they won't honor any insurance or warranty. They'll just blame it on the shipper and say the the shipper is an agent an not UPS. And you won't get a penny if your guitar is damaged.

I would get a large box and take the guitar in it's case, pack it around with clothing or bubble wrap and ship it unaccompanied baggage or use a different shipper.

If you are not confident in throwing it downstairs then you haven't packed it well enough.