I would only be ok with this after seeing proof of increased traffic and sales. Also, I would prefer it to be on more of a sliding scale vs. a set increase. Start at a flat fee per month and if you make X amt you pay a percentage of that up to a certain point. That way you can get more funds into the HC pot to keep up the cart and advertise but those shops that still struggle to make sales aren't drowning with extra costs and end up leaving HC, which would result in monthly money lost for the company overall.