The domain investor always prefers to use Amazon pay , because it is difficult to find change of 10 paisa or 50 paisa depending on the order amount to pay the delivery person. It is also easier to track the online transactions.
It appears that Amazon is very rigid in processing the subscribe and save orders , approximately 7 days before the scheduled delivery date.
If the balance is not available, the order is immediately converted into a COD, cash on delivery order
However the customer does not get any notification that the order is converted into a COD order.
Only when the delivery person is asking for cash, does the customer realize that the order is converted into COD order.
This can lead to arguments with the delivery person, why the order is converted into COD, despite having an amazon pay balance.