If you give your shoppers time to think, they can delay making purchases. Sometimes the delay will be short, sometimes it will be long enough for them to forget about the product and the offer altogether.
A very effective way of persuading users to make a purchase right then and there is by creating a sense of urgency. If you have any discounts or special offers in your store, you can always make them time-limited and notify your shoppers about it. This way, you will add a sense of urgency.
For example, if you have next-day shipping/dispatch add a countdown timer saying: "Want it dispatched tomorrow? Order within X time"
You can add those urgency triggers near the ,,Add to cart” button, on the top of the page or at the top of the product description.