Donorbox has a cover fee function where the donor can choose to cover the processing fee. To enable this feature, go to your campaign editor, and click on the edit button on the campaign you wish to edit:
On the next page, click on "Amounts":
Then, scroll down and enable the following option:
Once you enable this option, please click on "update", so the changes can be applied:
This will give the donor the option to pay for the processing fee during checkout.
Additionally, this option can be checked by default.
This is how Donorbox estimates the processing fee.