Discount codes shouldn't be highly usable.

edit: downvoted, sigh. ok. I'll elaborate. When you give me a discount code field on your checkout page, I automatically open up a new tab and Google around for "your site name discount code". If I can find one that works, you're just losing money for absolutely no good reason. I would've happily checked out without ever thinking of saving 5% on my $50 purchase. (c'mon, $2.50? That's not even a latte!) But, now, I'm obsessed, and I go back into 'bargain hunter' mode. If you're really unlucky, I'll find another site that undercuts you without a discount field, and just use them.

Save yourself the trouble. Apply a coupon via a cookie from a special landing page. I'll feel excited and grateful if one is applied for me at checkout and I won't even think of shopping around any more if I don't have one.

I take it you've never taken a customer service call with an irate customer who couldn't figure out how to use their discount code correctly. If anything, they need to be the most usable part of the checkout experience, believe me

So don't have a field. Problem solved.

What entitlement does the customer have to the discount - surely as long as it's possible to redeem it simply no one customer has a privilege to acquire it?

Customers will call after the sale asking to proactively apply discount codes.

Affiliates will call after the sale asking to proactively apply discount codes (so they get credit).

Customers will complain if a discount code has expired.

Customers will cancel, return and re-buy just to apply discount codes.

Discount codes suck.

(I think you mean retroactively not proactively)

haha yep!

You can take that view, often though that will mean the customer will just find a competitor that will work with them.