I need to know how much and in which way email usage for personal
purposes influences physical (snail) mail usage among the American
(USA) population.
I am interested in personal communication, not business or work related.

This does not give a breakdown of business mail/personal mail, but you
may find it interesting:

"InfoWorld Magazine reported on September 26, 2002, an International
Data Corporation study pegging worldwide e-mail message volume at 31
billion messages daily, projecting 60 billions daily by year 2006. The
final report of the President?s Commission on The United States Postal
Service states that the approximately 200 billion pieces of mail the
USPS delivers each year makes up roughly 40% of the worldwide mail
volume. Thus worldwide mail volume is approximately 1.4 billion
letters per day or 1/22 the global volume of e-mail."

http://www.postalwatch.org/pw2003/usps_v_flamingo_amicus.pdf