Twain does mention this in Roughing It, Chapter 25 page 199, with

On Fri, 2014-02-14 at 22:05 -0500, Robert E Stewart wrote:
> Help. Someplace I read, recently, that Orion Clemens sent an unsigned  
> invoice to Washington for firewood delivered by an Indian. Washington denied  
> payment.
> Sam then helped the Indian make a "very shaky" X as "his[X]mark and the  
> bill went through with no problem.
> Can anyone help me with a cite?
> Bob Stewart