"The Swerve" is a disposable razor designed for trimming the back of the neck and — erk — upper back in-between trips to the barber. (That is presuming you go for the full back shave at your barber. Personally, I prefer a more natural, winterized look, with festive garland braided in for the holidays.)
You can get a Swerve for $5 or $13 for a three-pack.
The Swerve Neck and Back Razor [ShavingStuff.com]