Well, assuming you don't get a place to enter your coupon code until checkout, that means you'd already decided that the listed price was worth it for whatever you were buying, right?
I often take a sale just to the checkout, to get an accurate picture of what shipping & taxes will be added.
"Shopping cart abandonment" is a specifically targeted "thing", and it's not unusual to get emails "reminding" me that I "forgot" that last crucial step of actually transacting.
I hate the is way of arguing. You don't decided that XX dollars is fair -- you decide that XX dollars, given everything you know, is fair. So XX dollars, if that is the best price might be okay, but XX dollars, given that there is a coupon or a sale next month is not a fair price.
I often take things through to checkout, even if I'm fairly sure I'm _not_ buying, as it's the easiest way to find out how much shipping is going to be added (this is especially true when I'm shopping on Amazon Japan or PayPal internationally, as going through to checkout is the only way to trigger the currency conversion).
Edit: It can also work the other way though - I've decided I'll purchase, see a coupon code box and think I might as well check (when I wouldn't have thought to otherwise), and ended up paying half the price (it was a monthly service though, and it made me less likely to cancel it when I wasn't using it as much, so they probably won in the end).
That's not necessarily true. I think most people, including me, assume that most retailers have some kind of coupon system and don't need to see a coupon field to think about coupons.
Anecdotally, there's been cases in the past where I see a product as valuable but overpriced, and a coupon makes the difference between a sale and a pass. I will go to the checkout screen with the product to try some coupons and see if they work, not having yet made the decision to proceed with the purchase.
