1. Tweet a promotion for a fee. Ex. “Just took dishes out of dishwasher and no spots. Thanks Finish http://bit.ly/Avpl3“
2. Background image real estate. Charge for a monthly rental, or even daily, for having your background image a flyer for an event or promoting a product.

The more followers you have the more you could charge.

What if there was a way to have a promotional line after an employees signature. So after:

Betsy Sue
Branch Manager
Company Ltd.
555.555.5555 ext. 555
123 Fake St.
Somewhere, Someplace
000 000

You had:

Our new optimax server is rich, see more.

And ‘see more’ went to some promotional page.
I wouldn’t see that as spamming or advertising, because you’re already dealing with the business. It’s just a simple way to promote.