Send them a message and tell them not to pay until all the auctions they have bid on have finished. Ask them to send a message when they are done. Once they've done that, select send an invoice from your seller page, which will show all the items they have bought. Adjust the postage cost in the invoice to reflect the discount. Make sure you work the postage cost out first. If the total including packaging is under 500g it will be the same cost regardless, but if it goes over 500g, use the Aust Post postage calculator so you don't undercharge.

DO NOT hit recalculate when you adjust the postage cost in the invoice. Change the amount to the correct amount, then send it. Don't forget to add a little extra to cover the fees that eBay charges on postage. I charge $8.20 for an under 500g parcel, which over the counter is $7.45.