Custom Sales Invoices - is this doable?

Would like to create a sales invoice that incorporates the following.

Invoice for outgoings at a set percentage
The original expense may or may not have GST.

I need to create an invoice that I can show the original price paid, Strip the GST if it has it, then charge a percentage if the exGST price , total and add GST.
Will email this invoice with a copy of the original paid invoice.

I have attached screenshot as an example.

Manager should be able to handle most of what you need without customization, though the format won’t be identical. Have you read the applicable Guides at to understand how to invoice on tax-inclusive and -exclusive basis? And have you read about using tax codes?

What Manager will not do is perform the proration calculations internally.

Okay thanks,
Will work through those and see If it works the way I need it.

If you are going to forward a copy of the original paid invoice with your invoice then you would be better off showing on your invoice entries over two lines - first line being a replica of the original paid invoice and the second line being the service charge + GST.

This way the Customer can more quickly reconcile the invoice without calculations.
Also, if you are going to have such situations regularly, then you might consider using the Billable Expenses module