Never used reversed text in sizes lower than 18 points. It will be difficult to read. If your direct mail letter has done a good selling job the next part is to ask the customer to do something. What a call to action does is ask the customer to do something now. Make a phone call, mail in the card, stop by the store, redeem a coupon call our toll free 800 number, visit our web site and so on.
Most people are more afraid of fear of loss then expectation of gain.
Or put an actual date when the promotion ends. Or perhaps a discount on the first 500 orders received. There must be a reason to make the consumer act now and not procrastinate. This is one of the most important parts of any direct mail letter. A percentage of your customers will go right to the bottom of the letter to see from whom it came.