GST rate for small Advertising Agency?

Presently, ST is charged only on 15% of the total bill amount, being Commission but on Media Space, which is 85% of total bill.

Under GST rate on Sale of Media space is 5%.

My query is: What rate small Ad. Agency should charge to its clients -5% for value of Media Space + 18% for commission by making 2 separate bills or 18% on total bill amount by making one bill?

Thanks