I noticed the other day that CustomInk uses the terms "voucher code" on their checkout rather than the more typical "coupon code".
This is a subtle language issue, and is likely too small to prove via formal A/B testing, but I think CustomInk is on to something important.
As a consumer, when I see "coupon code" smack dab in the middle of checkout, I feel obligated to break flow and open a new Firefox tab to google "retailer.com coupon" to see if I can save some money.
About 20% of the time this works, making it worth my time on average. Regardless of the cost-to-benefit ratio, I'd feel guilty that I was wasting money if I didn't spend 10 seconds looking for the discount after the site so loudly told me discounts were available.
CustomInk even reassures that having a voucher is rare. The screen says, "Enter a voucher code (if any):" Those eight characters -- the parenthetical "if any" -- are brilliant.
Because nobody wants to feel like a sucker. Nobody wants to feel they overpaid.
Here's example going the other way. The e text editor is an excellent product, but their checkout screen presents the coupon option right in the flow of checkout, as if having a discount coupon was mandatory. Recalculate, or you're probably overpaying!
(click to enlarge)
Their coupon code is marked "optional", but "optional" strikes me as more prevalent and commonplace than "(if any)".
Anything that distracts a shopper from finishing checkout isn't good for the retailer. Indeed, an increasing number of online retailers are intentionally removing as many links as they can from their checkout funnel, even dropping left site nav to remove distractions.
While the savvy shopper recognizes "voucher" as a polite synonym for "coupon", perhaps "voucher" avoids the flow-destroying, avoid-being-a-sucker obligatory google coupon search.