To mail a USB flash drive, tape the object between two sheets of cardboard before putting it in the envelope. Use stamps for large letter due to thickness.

https://auspost.com.au/content/dam/auspost_corp/media/documents/addressing-an-envelope.pdf

Small letters: $1.10 (no thicker than 5mm)

Large letters: $2.20 (no thicker than 20mm)

https://auspost.com.au/content/dam/auspost_corp/media/documents/post-guides/post-charges-guide-ms11.pdf

Add an extra $4.45 for registered post (tracking)