When using Send Jim you have the ability to "cancel" any pending mailings you choose and the credits will be immediately added back into your account.
Here are some screenshots showing you where that is done:
1: Go to the "Reports" tab
2: Set the "Toggle To Future Mailings" dropdown to "Yes"
3: Click the "Search" button to refresh the page.
Now you can cancel any order by clicking the "Cancel" button located on the far right side next to an order.
If you want to cancel only a single individual mailing to one person you will need to follow these same steps but first navigate to the "Tab" that says "Orders By Recipient"
You can NOT cancel a mailing that was submitted and set to "immediately" for the delivery. You can only cancel orders that have future mailing dates set.
PRO TIP: After you search for your future mailings you can sort the list by clicking on the "date" column located here: