Looking at the Virgin Australia Checked Baggage information page, I see that Pre-Purchasing an additional piece of checked luggage online costs AUD 35. However, the Excess Baggage Fees for extra pieces purchased at Airport is much higher, at AUD 70!
That makes me wonder - how close to departure can you go online and still be able to add an extra piece of checked luggage that way? Up to a day before departure? Up to 3 hours before? Or could you get to the airport, discover you're over your limit, and fire up a tablet/laptop/phone and add the extra bag then for the lower online rate?