A: J. C., Thanks for the question. Yep, you're right about customers buying from you. Cone Inc. has reported that 79% of consumers are more likely to buy a product that supports a nonprofit.
I'd start by talking to my current and potential clients about causes and nonprofits they care about. From this list, choose one, two or three charities--at most--that you can earmark for support. You can dovetail the interests of several clients by donating to a charity that is working to feed, say, starving children if they all share an interest in hunger relief and/or helping children.
With your type of business you can support these causes in two ways. First, you can simply make a flat donation and promote to clients that their business with you is helping your company support local nonprofits they care about. The second and preferred way is to donate a percentage of sales. Then you can encourage clients to do more business with you because that will increase how much you give to worthy organizations.
Couple more suggestions. Don't forget to go back to the nonprofits your supporting to highlight your generosity and to press them for business (I'm sure you'll broach the subject more delicately than I am). Explain that they can help themselves by doing business with you and by encouraging other companies to work with you. Also, if you come across a company that actively supports a charity not on your list, close the deal by offering to rebate a percentage of just their sales to their charity of choice. Stay flexible and make cause marketing work for the both of you.
Finally, there are lots of examples of small business cause marketing right here on Selfish Giving! On the right sidebar under categories click on Cause Marketing in Action.
Good luck and keep in touch. Joe