To add to what Mr. Divekar has mentioned, being one of the customers at the receiving end, and also a soft skill trainer myself, following things need to be inculcated.
1. In case the delivery executive has several destinations, he should know how to read Google map effectively so that he need not criss-cross between locations but plot his delivery points in such a way that he does not waste time and vehicle fuel.
2. He should have sufficient change even many customers would have paid on-line. I have noticed several times, the delivery executives saying 'sorry sir, no change'
3. Inculcate in them the importance of customer service, the ratings that you get determine future customers. Therefore, they need to be polite, soft-talking, apologise for short delays.
4. His vehicle should be in good condition and enough fuel to last for the day.
5. His dressing etiquette should be important such as the way he dresses, his appearance etc. Some people may say, so what, how does it matter,, etc. but if you need to enhance the brand image, it is necessary that the delivery boy or executive presents himself like a professional. It does make a change in the customer perception and please remember, it is a highly competitive world. Try to be one-up always.
Best wishes