The customer already has ECT running as a product purchase cart and it's working great.

He's now asking it there is some way to add a page through which an outstanding invoice can be paid.

Thank you for your help.

Sure, just repurpose the donations page into a make-a-payment page as outlined here - https://www.ecommercetemplates.com/help/donations.asp

Once you get that working, you can use this tip to send customers a link to auto-populate the fields, if you are so inclined
https://www.ecommercetemplates.com/support/topic.asp?TOPIC_ID=108615