Unless you want to manufacture for the B&M (and assume all liabilities included with that), the two options I would consider would be a licensing agreement or as a consultant.
With a licensing agreement, the B&M will pay you an amount up front for the recipe & production methods. Dollar wise it's pretty simple - % of projected sales - i.e. They project to sell 500 x 30ml bottles of your recipe per year @ $20 per = $10,000.00. Let's assume they make a 50% profit = $5,000.00. They buy the license from you for $1,000.00 per year. (This is just an example - numbers are all negotiated up front). There are plenty of sample licensing agreements on-line (Food recipe ones would fit this). No need to track sales, etc. Basically pay me up front, use it how you want; however, you can always negotiate in things like your name on the label, exclusive rights etc.
The other is they hire you as a consultant - call it a Flavor Consultant. 1,000 different ways to get paid - up front, monthly, 1099, $ per sale, or even some good ole fashioned horse trading for other premium juices, supplies, etc.
The main question you have to ask yourself is what do you want out of the deal? Do you want to become a e-juice manufacturer? Do you just want your name out there, do you want to invest the time making juice for them or is it just about the money.
I would recommend to start out with what you truly want out of the deal and go from there.
