For self-paying accounts and/or all kinds scanned documents, we've created the manual ‘Edit Line Items’ feature.
It is found in the bottom of the ‘Item Edit Page’.
Here you’ll find a green ‘Edit Line Items’ button. This will open up a moveable box in which you can create and modify line items, each with their own total, tax, category, project etc.
The window will also keep track of the sum of the lines to ensure that they match up to the total of the item.