My advice is to definitely negotiate, negotiate, negotiate. You should have an idea about what your limit is whether it is the % discount or the dollar value of credit before you begin and be sure to stick to it. From my deal and reading about others' experiences it seems to me that the Heartsy folks try to get high value deals in terms of the amount of the voucher. For me this just wasn't possible as I couldn't be on the hook for all shipping charges, so I chose an amount that would cover most items in my shop but not the total amount once shipping was added in. It definitely took back and forth with John before he accepted but I was very firm and upfront that if it couldn't be that way, then I would have to pass. I was completely prepared to walk away from the opportunity if it wasn't right, but I think explaining my line of thinking helped him to understand my strategy.
Good luck!
Kim