Hi,
There are two timelines.
1) One to ship the item which is maximum 7 days after making payment.
This is the timeline which the seller has asked you to extend.
2) The second timeline is the time for delivery after shipment for which a time period of maximum 10 days is allowed. Since the Seller has shipped on 24/08, the maximum time period allowed for delivery before which a claim for refund can be initiated is 04/09.
However since the product has been shipped in 24/08 by BlueDart courier, you can expect delivery within next 1-2 days much before you your need.
Devesh