I have seen one like this before as a legitimate "introductory" model. The price you get covers the labour, oil and filter of the service, and as the
OP said, you cannot provide parts yourself. It is a good deal if you have a well maintained car. The workshop does its nuts on the first service, but gets a chance if they find other issues with the car and will make its margin on the parts. They also have an opportunity to build up a client base.
The marketer keeps the majority of the voucher value, so you can see the gamble the workshop takes. SO it is in the workshops interest to impress from there. What I did see was that since the offer is "cheap" many voucher purchasers who took it up brought in cars that hadn't been serviced for ever, didnt read or understand the T&C's, and expected that the "service" would fix their car for $139. They then scream when they are quoted on what is required to just make their cars safe.