We just got an offer ...

My wife and I have received an offer to be "off-site" managers/consultants for the retail sales lobbies for a small chain of up-scale car wash facilities.
This kinda' came out of the blue, although we have had a previous biz association with the owner, working as independent contractors for a company servicing a line of products for his lobbies. I guess he liked what he saw in us?
We are hoping this may be the start of something, as we both enjoy retail, with 25 years experience as biz owners.
I also have been studying copywriting/advertising to possibly launch a biz writing career, so this may also serve as a sprinboard for this goal.
The owner has mentioned working on a "percent of sales" basis as compensation for our services, but we feel we would have to demand a "base-plus" arrangement as our set-up time could mount up.
Any suggestions/comments would be greatly appreciated.
Thanks!
Greg & Alice