I buy DVD mailers in bulk on eBay from eb-packaging; if I only need a few, I get the plastic* ones (151x229mm) from Officeworks (don't get the 151x229 paper ones or the Australia Post PB2 mailers, as they don't quite fit a DVD), which are just over $1 each if you buy in bulk (eBay is still cheaper by far, but Officeworks beats buying a PB3 mailer from the post office when sending only one or two items).
* If they give you stamps at the counter, put sticky-tape over them as they can fall off the plastic bubble mailers; the "parcel" labels do not have this problem. Learned that the hard way when I got a game sent back to me two weeks later sans stamps, and had to resend it. More than likely it went all the way from Melbourne to Sydney and to the buyer's local post office, only to come all the way back to me! Luckily, they received it eventually and were nice enough not to give me a defect.
For regular DVDs, Blu-Ray cases and games using the same type of cases (PS2/Xbox/GameCube and later), get the 160x230mm mailers. You can also fit two DVD cases in side by side in a large bubble mailer (even the AP ones) without having to pay Parcel Post (it's usually 5 stamps more often than not as it weighs over 250g, especially if the games have thick instruction manuals, but for DVDs or games without manuals it's usually only 3 stamps and weighs around 200g).
For regular (fat) PAL PS1 cases, it's Parcel Post no matter what; even if it fits in the letter gauge I don't trust AP with brittle plastic jewel cases which are pushing 20 years old (and no-one makes reproduction PAL jewel cases either, not even in China). Regular CD cases of course can go in AP's CD mailer, or PB2 bubble mailer for 2 stamps (or 3 depending on weight), but not many PS1 games were released in regular cases (mainly late edition Australian releases after 1999 or so e.g. Tekken 3 Platinum but not black label, and not Tekken 1 or 2).
Master System, Mega Drive, NES and N64 games (just the cartridges) will also just fit, but only if you don't have padding (e.g. use a tough bag). I have been stung Parcel Post on a Master System game at least once, turned out the width was 23mm thick and they refused to let it through for 2 stamps despite the whole thing weighing all of 80 grams.
Atari 2600 games and Super Nintendo cartridges are automatically Parcel Post as they are more than 2cm thick, as are VHS video tapes (audio cassettes - remember those? - fit but are of course very brittle, and I have no idea about records but I think they're too big anyway, notably the 33s, and you're just asking to have them snapped in half by the postal service if you send them in the mail as-is). Boxed cartridges are of course Parcel Post (the Sega games were basically miniature video cases anyway - Sonic & Knuckles actually used a standard VHS case).
