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You're writing the letter to thank your customer for doing business with you, so it's no longer necessary to subject the person to advertising. Assume good rapport at this point. Make the customer feel like an insider. Phrases like "we hope to do business with you again soon" sound like tag lines; best to leave these out. Don't say something you wouldn't say to an acquaintance. Don't include a product pitch, a mention of an upcoming sale, or anything else that could be construed as an ad. Even if you're mailing dozens of letters, it's better not to use a postage machine. That's a giveaway that this thank-you note is one of many, and it will make the customer feel less special. In fact, it may mean your thank-you note will end up in the junk mail pile. Again, the more personalized you make the thank-you note, the better it will be received. If you don't have time to address envelopes, have someone else do it. Even if you're not the person actually writing out the address, the customer will be impressed to see handwriting. Make sure your phone number and address are included in the correspondence, and give the customer warm encouragement to get in touch for any reason. If your customer does contact you, be ready to meet his or her needs promptly.
Resist the temptation to pitch your business again. Mail the letter with a real postage stamp. Address the letter by hand if possible. Provide your contact information and be open to communication.