On Moxie, a Flex Pay class is a donation-based or "pay-what-you-can" class. These classes are free to book and free to attend. At the end of class, students are prompted to leave a 'flexible payment' for the instructor, if they so wish or are able to. Students will see a suggested amount set by the instructor, but are free to leave any amount they wish (min. $5), or simply skip by and take the class for free.
You can also leave instructors one-time 'flexible payments' anytime by visiting their profile page and clicking on the Flex Pay button.
One-time 'flexible payments' are non-refundable.