Create a field specifically for entering a coupong / promotional code. Entering a code gives the customer a certain amount of discount depending on what code has been entered. On the merchants side, a number of different codes can be constructed in order to 1) measure where your customers come from and have heard of you and 2) allow different discount rates for different groups.
Using coupons codes to attract customers to buy a merchants product builds on the customer’s assumption that the offer is short lived, why action must be taken soon in order not to loose the psychological gain given by receiving the coupon code. As losses are mentally valued with greater weight than gains, the customer will be aversive towards losing the gain given and act on it while time is. Check out the Value Function as proposed by Tversky and Kahneman, 1991: Loss Aversion in Riskless Choice.
Another benefit of Coupon codes its traceability. Marketeers can branch out different codes to find out which campaign generated the most leads.
You can fill out a coupon code when buying at amazon.com
At first glance it is hard from the design of the shopping cart, to connect the entered coupon code's effect with the actual discount given.
Dropbox.com lurkes new users in by offering more space for each new referred customer.