Not sure how to charge for ppc management

Hi Guys

I am new to the forum, so I will thank you for your input ahead of time.
I am a new ppc manager. I am working with a small local company that sells central heat and air units online. I need to tweak their offering as well as their website but my big question is should I base my charges on a percentage of the gross sale per unit after Google fees or on their net profit per unit ( which of course is allot less )? I really need this account as it would be my bread and butter.
Again thanks in advance.

This a duplicated post of this question.

Anyone have any advice?

I personally prefer a flat rate agreement for the first year, until it's worthwhile to switch to a % of revenue generated.

Most SEM companies charge either a percentage of ad spend (which I personally consider a conflict of interest because it is an incentive to spend more instead of wisely) or based on the number of keywords in the account.

I believe paying for results is the best path; however, few advertisers are currently paying based on percentages of profit.

I would suggest that whatever is fair for both you and your clients is the best arrangement.