Testing your product isn’t only about user interface – where a user gets confused, and so on. It’s also about uncovering niggly little problems like this:
They aren’t the only ones. Here’s Amazon, which has free super-saver delivery in the US for orders over $25:
This is an opportunity for an online store to delight users by saying, “You haven’t quite met the minimum spend for free delivery, but what the hell – we’ll give it to you anyway.” The sale goes through, users were pleased that you were nice to them, and they’re more inclined to come back next time.