It interrupts the purchase process
Once they reach this step in the order, the customer has pretty much committed to buy the item.

Can cause the user to leave your site
The promo box tells them to google a promo code and try their luck finding one that works. While searching, they may get distracted and abandon the cart.

Loss of $
If the customer does find an online coupon, you as the retailer will loose $ on something that most likely would’ve been sold as list price.

Zappos does a great job dealing with this. The ultimate killer is that they don’t offer coupons. They also made a page (with a great title) explaining why they don’t do so. This of course is the #1 result when searching google for “Zappos coupons”.

Welcome to Zappos.com. Many other sites out there may claim to have Zappos coupons, but the truth is Zappos does not offer coupons. Zappos.com is based on service and selection, not price.

From Zappos coupon page. If you do offer coupons you can get rid of the box and just stick the promo info in the url or add an email form next to it so you can capture addresses. Say something like, “Enter your email to receive promos”.