Medlife - Cancellation of PO by MedLife after the acceptance of payment and without my prior consent

A purchase order has been cancelled by MedLife after the acceptance of the PO and the payment without my prior consent.
After a successful online payment I received message from MedLife that the items would be delivered on certain time and date then they sent me another message in which they postponed the time of delivery and then I received two similar messages informing me about the changes in the delivery time. They later informed me that the delivery is cancelled due to the nonavailability of the required medicines.
Why have they accepted the PO prior to verifying the availability of the medicines?
I have already paid the amount online. I did not even get back my money so far while I received a message from them that the money is successfully refunded. They have not even sent me a refund slip