E-commerce website and GST

I have an online women's apparel store. Lets say its ABC company. The products that will be showcased is of a manufacturer in Mumbai who said that he can send the products directly to customers (Drop Ship). Lets say that manufacturer is XYZ Company. If a customer orders from my website then i am going to put forward the order to him and he will dispatch the product to the customer. He also said that he will offer me 20% discount on the retail price of the products. I am new to this and I want to know how GST works on it. If I sell on exactly the retail price how would the GST work in this case. How much GST will I have to pay and how much will he have to pay? How would the invoicing be done? If I want to send the invoice to the customer directly how can I do it? He said that he cannot send the products without a bill as the courier company don't sent the courier without bill. So how will the invoicing system work between me and him and finally the customer as he will offer me 20% discount and i am going to sell to customer at retail price and not offer the customer the discount. Probably the discount offered by XYZ company will act as my profit margin.