I had no idea those options - Credit, Write off, FX and Discount - even existed... and I've used CB for years! I've seen the options link thousands of times, but never thought to click it! I've even manually done write-offs against bad debts: this option would have certainly been easier.
A quick idea that may help users be aware of these options: how about always showing the options, along with Quick Pay, as a 'tabbed' section, comparable to the Manage money page under Money | Bank accounts. Here's a quick mock-up, with Credit selected:
The Quick pay option would remain the default option, but the other options being permanently visible would make them more accessible. Also, it would be good for the browser to remain scrolled down to the Payment history section when an option is clicked; at present, it jumps back to the top which is a bit confusing.
I'm happy to repost this in the Ideas section if that helps!