I just went through an experience with a business owner that may help you with your process...
First, I agree with bmcgee...ask for a flat rate per sale up front (gross)
Make sure you discuss this thoroughly with your business partner about what he is wanting to make per sale (what is his profit margin)..and what he can afford to pay you per sale.
The next thing to hone in on is the actual market dynamics for the particular products or services that are being sold so you can guestimate your actual income from your efforts.
Is it cyclical and sees big activity during certain times of the year? ...or is it something that will get very steady order traffic...?
Regarding tracking of phone/fax orders...if you can get the consent of your partner to set up your email address as one of the fields that gets information per transaction from your partner's credit card processing service (like Authorize.net) then you will know precisely the orders that are coming in via fax/phone if they aren't finding their way into the Web site database....better yet make sure you have access to this via the user/password (if your partner will agree to this) so that you can regularly check these.
A certain percentage of people refuse to place online orders and will only use the phone/fax to do so....so be sure you can access this and put it writing how this will be done.
Make sure you set a regular, accountable day once a month when you will recieve payment and don't let someone dictate how many sales are required before this is active.
Also, see if you can negotiate a percentage based annual bonus (say 3% of total sales ... here's where net may come into play...for the any calender year)..
Make sure your partner doesn't have a gambling problem and is pilfering away your hard earned dollars and not able to pay you .... or payments are delayed for weeks on end...
very painful to discover something like this after you have worked so hard...
Absolutely state infactically exactly what your responsibilities will be....this must be spelled out to the letter or you might find yourself being sucked into becoming a sysadmin or something you didn't bargain for and stick to your guns on this....
Well, thanks for letting me purge that some...send me an invoice for the counseling session..