A 35mm box will force you to pay Parcel Post costs unfortunately, as it is over the 20mm limit Australia Post has put on regular mail. I gather these boxes are similar to the VHS video boxes that both Australia Post and Officeworks sell. Buy some 160x230mm bubble mailers (padded bags) on eBay instead, they are much cheaper than the cardboard boxes. I recently bought 200 of them for under $30 with free postage. I have sold close to 1000 games (although hardly anyone leaves feedback these days so my feedback of 514 doesn't show it) and am yet to receive a single complaint about an item damaged in transit.

A standard sized DVD case in a bubble mailer fits through as a large letter at 19mm. Anything thicker than a DVD case will most likely be considered a parcel and it will be $7.95 and come with tracking automatically.

DVDs (and games without manuals) usually need 2 stamps while games with manuals usually need 3 stamps. Blu-Ray cases (and PSP, Xbox One, 3DS etc.) are lighter and usually only need 2 stamps but it's best to weigh everything. 0-125g is $2, 126g-250g is $3, 251g-500g is $5.

Tracking isn't very cheap unfortunately, Registered Post is only about $1 or $2 cheaper than Parcel Post these days as they made sure to push up the price of the RP stickers to almost match parcels (and then put up the parcels another 35c anyway!). The cheapest you will get is $6 on a DVD, which might be worth more than the disc itself!

A game with $3 worth of stamps plus a $4 Registered Post sticker becomes $7. Add another 50 cents if you are also sending it as priority mail and it becomes $7.50.

I also put a small amount of bubble wrap inside the DVD case to protect the disc. I cut 5x10cm or 5x15cm strips and fold the long side two or three times to make a cushion with the bubbles on the outside. If the disc still moves with the case closed, fold the bubble wrap again until everything feels solid.

And yes, you cannot print letter rates via eBay, you have to post them the old fashioned way using stamps. Australia Post hasn't offered a 250 gram parcel rate for close to a decade.

Speaking of 250 grams, do not ever send a 250g large letter (5 stamps) as Registered Post, that would cost almost as much as a 3kg satchel!