When you have PayPerMail enabled to work in your Magento backend, you can select this method when creating an order. Although currently, when only 1 backend payment method is active, no selection box is shown. Like shown on this image under "Payment Method". This is a core Magento bug and you can work around this using a temporary fix like discussed here: https://github.com/magento/magento2/issues/11380
Enable a second payment method that is active in the backend when creating orders. When there are 2 options, it is also possible to select PayPerMail from Buckaroo.