Right now, users have to click on text that says 'Have a coupon code?' on the payment page before the field where they enter the code becomes visible.

I've had multiple customers who've had difficulty with this and abandoned their job posting. The reaso for giving coupon codes is so that they can see how easy the site is to use and that they'll have a good experience, but they're actually exxperiencing difficulty.

It would be better if the field were always visible (other sites do this, so I don't think there should be any problem).