Why are my shipping charges reduced on final invoices?

[Fairyleonie]

Help! ive had my shipping rates set for years .

I haven't changed a thing-

Recently I noticed that the orders ive been receiving have had reduced shipping charges which I did not make.

for example regular postage for a 500g Australia Post satchel  I charge $12 but the customer only paid $10.91 on the final invoice. I went through the process of "ordering" online and its coming up at $12.

Is Shopify taking gst off the shipping. Is there something I need to change ??

im a little flumoxed- doesn't seem a lot but every dollar counts

thanks in advance