First and foremost, as a HR professional, you need to understand the difference between "Offer Letter" and "Appointment Letter". Are you clear about the difference between the two? Once employee joins an organisation, employer needs to issue appointment letter and not offer letter.
Karnataka Shops and Establishment Rules, 1963 mandates issue of Form Q which is also called as "Appointment Order" to all the employees. Find out a form that is equivalent to this form under the provisions of shops and establishment act of your state. Issue of this form for the level of delivery staff will suffice the purpose.