Upvote:1
I could not find a general rule valid across all carriers. In fact, this issue seems to be regulated by carrier-dependent terms and conditions.
Qantas allows you to pre-purchase additional baggage allowance from the time of booking up until 3 hours before departure, provided you did not check-in already. As stated in their FAQs:
Additional Baggage Allowance may be purchased directly after booking up to 3 hours before departure (provided the passenger has not checked in for the flight at the airport).
Emirates, on the other hand, allow you to purchase extra baggage allowance from 72 hours to 90 minutes before the departure time. Here is the text from their FAQ:
Purchase may be made between 72 hours and 90 minutes prior to flight departure and is valid only for the flight and date shown on the excess baggage purchase receipt.
Unfortunately the Virgin Australia term and conditions of carriage do not mention the purchase of additional baggage allowance. I would nonetheless speculate that you are allowed to pre-purchase extra luggage up until you check-in.