I am setting up an invoice system that will have different services attached but some of those services need to be related. I find that if all services are related to the invoiceID, I will not get a proper 'A' description when service B is a child of service A.

If B is must be related to A should it only be related to A and not the invoice. Or would you keep it related to the invoice as well as a second relationship to A?

Yes, relate A to the invoice and B to A only (not the invoice).