You can add an invoice to a supplier by completing a purchase order. This will make creating the invoice much quicker as it will already have the correct items, quantities, and prices. You can edit the invoice before you complete it if you need to add more items.
How to add an invoice from a purchase order:
Click 'Suppliers' on the navigation bar to open the supplier tab.
Click 'View all suppliers' in the top right corner and select the supplier from the list that you want to add an invoice to.
Click 'View' next to the purchase order you want to create an invoice for.
You will now see the details of this purchase order. Click 'Complete purchase order' in the top right corner.
This will take you to a page where you can add an invoice number, (if you have this automatically set then you will not need to add an invoice number) you can also add/edit items in the invoice if you have more parts/items that need to be paid.