The most simple method that does not skew the result of your profit margin consists of:

• Make an invoice at $0 for your client.
• Make a second receivable invoice for your supplier.

Client invoice:
Make the following invoice at $0:

JR    Replace alternator – 1 hour, with a cost price of $70/hr.
JR    Replace alternator guarantee – 1 hour, with a cost price of $70/hr.
AL    Alternator with a quantity of 1, cost price of $70 and sales price of $155.
AL    Alternator guarantee with a quantity of -1, cost price of $70 and sales price of $155.

Supplier invoice:

JR    Replace alternator – 1 hour, with a sales price of $70/hr (depending on whether the supplier pays your labour).
AL    Alternator with a quantity of 1, cost price of $70 and sales price of $70.

NOTE:
If you put a cost price of $70 and a sales price of $0 to make a guarantee invoice, you will considerably lower your average.