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@treasuress.4207 wrote:

When combining items, often it's more economical to use a satchel, which reduces the cost, however this isn't always the case unfortunately.  With the items you have bought, it is actually more economical to post them separately, as combining them would actually increase the shipping costs, thanks to Australia Post.

 

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We use similar.

You could add a cut and paste from the Aus Post site:

A large letter can be up to 360mm x 260mm in size, 20mm thick and weigh up to 500 grams. Anything larger than this is a parcel.