Type size may not be the most exciting element to test on your website, but losing sales because visitors cannot read your copy is definitely a downer. Unless you have a narrow target audience of people in their teens and 20s, legibility is something you don’t want to overestimate.

In this week’s Website Testing Win, an online retailer of specialty products recognized that its audience was likely to have failing eyesight. While the majority of its website design featured copy that was larger in type size than what you find on most websites today, it was concerned that the legibility of its toll-free phone number for Customer Service didn’t pass muster.

To find out if it could improve the legibility of this information—which might enhance visitors’ comfort level on the site and possibly lead to an impact on performance metrics—the retailer tried bumping up the type size on this text by a point or two.

As a result, type size wasn’t the only thing that grew. Customers responded to the change with a 1.24% lift in average order value, which sounds small until you learn that it represents a six-figure addition to the retailer’s annual revenue.