Use for services with variable fees

I am just assessing this plu-ins viability to use for billing clients.

Ideally I would create a proposal or invoice of work, once approved they then pay.
Each would be a different amount.

Would I create single products for this?
Is there a differeent way of doing it?
or would I create add the custom code to custom content type?

any views or am i trying to put a square peg in a round hole?