What if you just played the "honest" card with them? Be honest and tell them that you haven't gotten any sales yet but that doesn't mean that you won't ever. Bring up the fact that if you have there brand on your page, you aren't charging them. What I'm trying to say is that they are paying you $0 to have there products on your page, so why would they say no? Yes you make a commission when you get a sale, but that should be expected for them to give you money then. Hit me up if you have any questions!