Mypost Business is currently offering 10% discount for a 2 month period, so $7.425 + satchel. This will continue if you average 5 parcels per week. Bigger discounts are on offer for higher volumes. I have no idea how it works for own packaging, but it is presumably similar, so $7.95 less 10% equals $7.15.
I buy AP prepaid satchels from eBay sellers when I get a discount voucher. My last lot cost $6.50 each, and I didn't have to lay out $750, so there are other ways of minimising stock and postage costs.
