Great question! You can create special offers from My Account > Special Offers. You have the option to limit the number of purchases or set an expiration date if you want. Save and activate the offer and then send the offer URL to the customer. You could also create one special offer for this that doesn't expire and include that offer URL in the message to customer so you don't have to do any followup.

Also, in case you don't know, you can create special offers and then submit them for the Deal of the Day (just another button to click after saving and activating an offer). I tend to schedule Deals of the Day during the last week of the month for the following month. And you can even include other developers' add-ons and themes in your offers--they get an email to approve the offer before it can be active, of course.