Amazon subscribe and save prices are changing

It appears that the prices indicated in Amazon subscribe and save are changing over a period of time.
For example in September 2018, the domain investor found that the amount payable was Rs 410+ so she added the required amount.
However after a few days she got the notification that she still had to pay rs 12 for the order
That day, the domain investor had some other urgent work, so she could not immediately add funds
She added funds the next day, however the order was already processed and the account was not debited for a particular item