[swombat]

Wow, a generalisation based on a single anecdotal data point!

You could sink the titanic over again with holes that large.

Guys (and gals), please resist the temptation to take a single personal anecdote that you care about and blow that up into some kind of authoritative advice such as:

Do not show the Coupon code field unless you absolutely need to do so. When sending marketing and promotional materials, send them to a different version of your payment page that reflects the discount you are offering. Having the same payment page for your discounted and full price purchases just invites Google searches for “(app name) coupon code” and resulting abandoned cart.

Those arguments are no better than http://xkcd.com/605/

[nhangen]

I've canceled several purchases for the same reason she did. Granted, if I need it, I buy it, but when it comes to discretionary purchases, the box has made a difference.

Maybe it's anecdotal, but add enough of them together and you have a legitimate case against the coupon box.

[z0r]

You can't sum anecdotes, addition is undefined for that kind of data

(But there might be a case there if you took some real measurements)

[nhangen]

You get the point. Sentiment matters.

[swombat]

The plural of anecdote is not data.

[dfxm12]

You can add another point to the data set. Granted, I don't crash my browser looking for coupon codes, but I always try to, when I see a check out page like this.

The other issue to me is that everything else on that page is required except the coupon code. It really makes me feel like I'm missing something.

Remember, this is the World Wide Web, so any claim you make is a gross over-generalization because, by and large, you don't know who your customers are going to be. You can only do A/B testing, collect feedback from your customers, etc, to slowly get better as you go along. If the author is in the minority, who cares? If not, we'll start to see different types of check out pages and the ones that will "win out" will be the ones that meet a sweet spot between ease of implementation and effectiveness.