How much does font-choice matter? Check out this study reported at Futurelab: The Wrong Font Can Kill You. Literally. Your Sales, Too.
In a study of patients, when patients were given medical instructions in harder-to-read fonts, they perceived the instructions as difficult. The difficulty of the font made the information more difficult. They were less able to follow the instructions.
Are you doing that in your fundraising?
If you're using hard-to-read fonts, or other stupid design tricks like reversed-out type, type over color, type over images -- you're making your material "more difficult." Even if the writing is perfectly clear and frictionless -- it seems to readers like legalese or IRS forms. They are less able to understand what you're asking.
Some designers and brand experts choose fonts for how they look and feel, with little regard for how readable they are. Getting that look just the way they want it is more important to them than readability. That's bad design, bad communication, and bad fundraising. Don't put up with it!
You can still raise money with hard-to-read copy. Some donors are determined to do the right thing, no matter how rude we are to them. But you'll get less.