The easiest option for items that you are not sure of is to take to the PO and they will help.

If you are going to continue to sell here it might pay you to get the Free Postage Booklet from AP that is available over the counter. 

It has everything (and more) that you could want to know. There is a lot of info that you may not want, but your info will be in there too, and it will remain as a phsical reference for you.

A new copy is released every 6 months, usually Oct and April so just ask for the new one when due. 

You might benefit from a set of Digital scales so that you have an accuracte weight. We have a pair that we bought at K-mart for approx $19.00 12 months or so ago and is a huge help. Don't take up much room, and we just keep in the cupboard for when needed.  

Also If you are selling here on a regular basis, would pay you to invest in some "tools" to assist you in your listings.

Scales being one, the post booklet, and we also have the free Letter gauge from AP, small scales for weighing jewellery, and a few other things collected over the years.

Putting these items together will make your selling experience much easier, and less stressful. 

Good Luck