I version 4 we had the ability to add a task to an invoice that was already created. This helped us avoid having to create multiple invoices for the same client for different tasks. This was especially helpful if an invoice was already sent to the client but had not been paid for, thus allowing us to just tack the new task to that invoice.
Hi,
This option is still available in v5. If the invoice and task have the same client you can click the plus in the lower right corner of the edit invoice page to add existing tasks to an invoice.