Something that I have noticed in the last year is "not everyone reads their e-mail on a daily basis". For those that do not have e-mail at work, there are many that check it "every once in a while".
As I do a lot of client correspondence via e-mail I also e-mail with 'read notification' to prevent sending a reminder when they haven't read the original (be it an invoice or mail).
And personally, I tend to stack my bills (paper version) in one spot to pay on one day, so an e-mail invoice is going to have to get printed to add to the stack. "Oops, I forgot..."
Of two operators I know doing it - one does 70% (they work only commercial properties) and one does less than 20% (mixed). And each of them try to get e-mail addresses as a standard business procedure for new accounts- for news, invoices, corresp., etc.
Definitely saves the sender the postage!
For those considering it in the future, Quickbooks has the feature.