Add service charge percentage to multi vendor website (Wocom)

Hi, i run a multi vendor website that works similar to www.justeat.com.

Each vendor runs and sets up their profile and adds their own products for my customers to purchase.

I would like each vendor to be able to charge a certain % servicing charge fee and this is added to the customers bill in the checkout.
So Restaurant 1 could charge a 2% service charge fee on their products and Restaurant 2 could charge a 10% fee on their products.

So if a customer add products worth £100 to their cart they will pay a total of £102 in restaurant 1 or £110 in restaurant 2.