Very simply, a licence is the legal way for you to give permission for something to happen. Offering a licence carries some weight in that if they break the terms of your licence then you would be within your rights to bring legal action(s) and restrict the use of your work.

If the intent is to offer a commercial licence at an additional charge then it is more likely to cause confusion than appear as an additional/better offering. Getting those who are outside the industry to comprehend copyright is hard enough as it is; there's no need to make it more difficult for yourself.

There is also a question as to if your business would constitute work-for-hire, and I'd say it does NOT. The Copyright Act, Section 101 defines 'work for hire' as one of two categories...

A work prepared by an employee within the scope of his or her employment

Which would not apply to your situation as a freelancer. Or...

A work specially ordered or commissioned for use

1. as a contribution to a collective work,
2. as a part of a motion picture or other audiovisual work,
3. as a translation,
4. as a supplementary work,
5. as a compilation,
6. as an instructional text,
7. as a test,
8. as answer material for a test, or
9. as an atlas.

If the parties expressly agree in a written instrument signed by them that the work shall be considered a work made for hire.

While there may be some wiggle-room for your work to fall under the guise of a supplementary work or a revision to the work they've supplied you and it is agreed in advance, a great deal depends on how the courts in your jurisdiction those terms.

Clearer is usually better from a legal perspective. As such I would advocate that you treat all your output as derivative work and grant an appropriate perpetual royalty-free licence that reflects this. Doing so also gives you explicit control of your work should they decide that they like the artistic decisions you chose as edits enough to use them (or reproduce them) but not enough to pay you for them.