"So basically, the affiliates are claiming commission on a sale that was going to happen anyway"

You have someone in your checkout process. Why on earth would you give them a reason to leave that process and go search for a discount code if you know they won't find any? Assuming they will come back doesn't feel right. What if their search for a discount code didn't prove there wasn't one, but did prove a competitor was going to be a) cheaper, b) a better service, c) offering discount codes, or they got distracted by cat videos on YouTube.

And maybe I'm naive here, but since adding affiliate marketing, if the revenue was going to be guaranteed anyway, wouldn't there be a drop in $200,000 from the regular revenue?