Yours is not a case of mixed supply. Example quoted by you attract GST @ 5 %. You are selling a single item.
Relevant extract of Board's Flyer No.4 dated 1.1.18
Mixed Supply
Under GST, a mixed supply means two or more individual supplies of goods or services, or any combination thereof, made in conjunction with each other by a taxable person for a single price where such supply does not constitute a composite supply;
Illustration : A supply of a package consisting of canned foods, sweets, chocolates, cakes, dry fruits, aerated drinks and fruit juices when supplied for a single, price is a mixed supply. Each of these items can be supplied separately and is not dependent on any other. It shall not be a mixed supply if these items are supplied separately.
In order to identify if the particular supply is a Mixed Supply, the first requisite is to rule out that the supply is a composite supply. A supply can be a mixed supply only if it is not a composite supply. As a corollary it can be said that if the transaction consists of supplies not naturally bundled in the ordinary course of business then it would be a Mixed Supply. Once the amenability of the transaction as a composite supply is ruled out, it would be a mixed supply, classified in terms of a supply of goods or services attracting highest rate of tax.