Usually the cash paid to the delivery person in excess of the bill amount, is pocketed by the delivery person as a tip.
However it appears that for Amazon delivery persons, they are supposed to credit the amout paid to the Amazon balance of the customer.
so if the amount is even less than one rupee more, the amount is automatically credited to the account
For example for an order of 98.10 , which was automatically converted into COD by the software, despite there being enough balance
So when Rs 99 was handed over to the delivery person, .90 was added to the Amazon balance
A sms notification was also received.