We have added the ability for races to have a quick donation on the checkout page. This appears right below the credit card number and can be set for a single charity and and single $ amount as shown below:

If checked, it will automatically update the checkout amount:

To set his up, you must have at least one Charity set up under Donations. You enable it with a simple checkbox, set amount, and pick the charity (if you have multiple):

If you do not want to have donations as part of the registration process, and only show this option at the end, then set your race up with only one charity and enable this option. The donation page will still show as a direct link from your race website.
This is meant to be simple and quick – hence no choice of amount which charity by the participant. Also note that this option will not show if the participant has already made a donation during that step of the process.