tapasfun You will be in loss if you work in less than 20% you have to convince people and tell them why you are charging those percentage. To me a business is not successful unless its earning profit may be people will let me look at Zomato and Swiggy but they are burning Investors cash out there.
For shared hosting you can look at Siteground.
Business is art of convincing people let it be vendor or customers.
I will suggest make a commission tire level 1 -10% level 2- 15% and level 3- 20%. Now tell them the difference between these three tire level 1 wont have the featured tag meaning they will be visible less. In level 2 you will provide offers from your end and will bear the discounting costs. level 3 will include social media marketing for that store in your social handles also you will push store specified notifications meaning there will be more chance when a consumer click on notification they will end up ordering from that particular store.
When you will tell these benefits see how many vendors wants to opt for level 1 and not level 3.
Right now they don't have plans to differentiate. Don't forget restaurant business have more than 60% profit margin.