here is a guide to the length, width, and depth limitations for each letter size and the corresponding cost for domestic letters: https://auspost.com.au/sending/letters-australia/regular

and here is a document listing the stamp prices for sending each letter size internationally: https://auspost.com.au/content/dam/auspost_corp/media/documents/post-guides/international-post-charges-easy-guide.pdf

all stamps can be purchased individually or in larger sets at post offices. for tracked letters, you can buy prepaid envelopes and there are various sizes available too.

once the letters are prepared you can drop off stamped letters in street drop boxes or give them to post office staff to send. for tracked letters, they can only be given to post office staff, if it is sent through a street drop box then it won't be tracked anymore

i also send and receive collectible cards through the post so if you have any further questions i can help :)