The telephone, telegraph, print photography, fax machine, SMS, Internet, photocopying, and computer-generated printing all had an impact on mail volumes for correspondence. (Photocopying, computer-generated printing, and print photography actually boosted volumes, a lot. The telegraph may have boosted volumes too because there was a paper copy generated, and often there was follow-up correspondence.) And all those technologies probably helped boost package and parcel volumes with a few exceptions like audio and video recordings and books, now often digitally delivered over the Internet.
But note how the U.S. ended multiple postal deliveries to residences in 1950, before most of those inventions. (And the 1950 telephone wasn't yet too much of an alternative for long distance communication.) The primary reason is probably due to rising wage and salary levels, and most people would say that's a good thing in a country's development. However, I think local telephone service probably had some impact, too.