Billit lets you add an extra description to your order line. To activate this feature, follow these steps:
1) Go to Settings > General > Advanced.
2) Activate the 'extra description for order lines' feature.
3) Go to Settings > Corporate style > invoice. In the editor on the right, go to 'Line', add $Line.additionalDescription$ under the key $line.description$ and save the change.
When you create an invoice, an extra icon will have been added at the end of the order line. Clicking this 'sheet icon' will make the 'extra description' order line appear.