Shipping charges are set by Australia Post.
Australians can benefit from this as they will have the lowest shipping costs.
UK, Europe and US customers will pay more for shipping, but with the exchange rate there is a good deal to be had.
Unfortunately Australia Post does not send to South America, Africa, West Asia, India and Eastern Europe.
A4 certificates are posted letters
A3 certificates are posted as small parcels, hence the higher cost.