I've sold my first item. How do I chose packaging

[rodri-shell]

Hi there,

I've sold my first item and listed it with the Australia Post 3kg Satchel as the form of postage.  I now want to use the "create postage label" to send it out.  Do I go into the Post Office and collect a Satchel to put it in and attach my label to that?  Or do I just use my own packaging?

Also, if I listed it with the 3kg satchel as the form of posatge, but then decide to put it in a box, is that ok? 

Thanks!

Shell