Import billable expenses to sales order invoice

I have billable expenses for a customer. And under sales order i have also created sales orders for that customer. By the end of month i want to create a sales invoice for the customer. But when i clicked to create sales invoice for the sales orders i created, it only shows the sales order items, there is no way i can add/import the billable expenses of the customer to the same sales invoice. These billable expenses are related to these sales so i do not want to send a separate invoice for them.

Is there any way i can combine this billable expenses in the month-end sales invoice?

Yes, there is.

Read the final major section of this Guide: Use the techniques in it to add the billable expenses to the sales invoice copied from the sales order.