Orders can have three different states, Unfulfilled, Fulfilled and Partially fulfilled. The idea is to keep track of what orders still require action by the shop owner.
Unless you wish to do additional work on your donations, it might be easier to enable automatic fulfilment from the DonateMate settings.
Automatic fulfilment takes the work out of marking donations as fulfilled. As soon as the order is marked as complete, if a line item is from a DonateMate product, it will automatically be marked as fulfilled. To avoid large numbers of emails to your customers, no email about the fulfilment of the donation will be made.
To enable automatic fulfilment, go to your Shopify Admin > Apps > DonateMate, then select Settings and tick the box for Automatic Fulfilment. Don't forget to press Save
If you have any questions, drop me a line at firstname.lastname@example.org