I suspect that Times New Roman is also a simple enough font -- for a
serif font -- and it would probably wouldn't give the USPS any
problems.
But you're right about Helvetica. It used to be one of the most
popular fonts in the world -- and last year it celebrated its 50th
anniversary -- until Microsoft came up with Arial in 1992.
Here are a couple of paragraphs from a piece called "The Helvetica
Hegemony" at
http://www.slate.com/id/2166887/?GT1=10034
"...Helvetica has weathered the transition from lead to digital type
remarkably well. Since 1985, every Apple Macintosh computer-the
choice of most graphic designers-has included Helvetica as a
"resident font," meaning that Apple licensed the typeface and
embedded it in the system's software. Microsoft, on the other hand,
looked around for a cheaper alternative. In 1992, the company chose
Arial, a digital Helvetica knockoff, as the default font in its
Windows software. Since then Arial has spread like a virus, much to
the dismay of graphic designers who dismiss it as a homely imposter.
To an untrained eye, the two typefaces are nearly indistinguishable,
though there are a few telltale differences: Helvetica's lowercase a
has a tail; Arial's doesn't. Helvetica's R has a curved leg; Arial's
is straight. Helvetica's G has a spur on the bottom right; Arial's
doesn't. (Designer Mark Simonson offers a more thorough comparison of
the two typefaces here. Think you can tell them apart? Take the quiz
here.
In what you might read as a tacit acknowledgment of Arial's
inferiority, Microsoft chose Helvetica for its own corporate logo..."