As a commercial pilot, can I fly my spouse for her work? So I hold a commercial license and have a 2nd class medical.  My wife is an engineer and a product she works on needs to be altitude tested (unrelated to aviation). She was suggesting that we could rent the plane from the flying club I am a member of and she could submit the receipt and get reimbursed for the flight cost. If she wasn't my spouse and was a friend asking me to rent a plane and give her the bill I'm pretty sure this is pretty clearly not OK.

We talked about doing a "pro-rata share" but wasn't sure how that's defined cause... it's my wife - shared finances and all that.  Any experts have any feedback on whether or not we're okay to do this?  Or if we really want to do it do we just eat the cost of the flight ourselves?

EDIT: Clarifying based on comments - This is USA (CPL) and my wife and/or company wouldn't be on the bill. It would be my name because it's a flying club and they would only rent the plane to me.  So it would get charged to a credit card in my name (joint account with my wife) and she'd turn around and get it reimbursed.  I agree, I'm not "holding out" in my opinion... but just trying to make sure I keep things above board.
The prohibition you're worried about as a CPL is "holding out" flight services to the general public, which can be summarized as providing both plane and pilot as a package to strangers.

In this case, you would be providing both plane and pilot, so your concern is understandable.  However, there is a clear personal link from you to your wife and via your wife to her employer, i.e. they're not the "general public", so you're okay.  It would even be okay if your customer were a bona fide friend, rather than your wife or her employer.

If the company could rent (or otherwise provide) the plane and separately pay you to fly it, you wouldn't need that personal link.  Given you're renting from a flying club, that's not possible, but it's something to keep in mind for future scenarios.  Do check with the flying club to make sure it's okay with them and their insurer.  I assume you have the appropriate insurance for yourself.

The pro-rata stuff only applies to a PPL, and given the FAA's policy that flight time and even employer goodwill can count as "compensation" even if no money changes hands, a PPL could still get busted even if they "eat the costs" as you propose.  As a CPL, though, you don't have to worry about that, just the "holding out" rule.