I just sent an inoice with postage by Click & Send for $11.50 as it is a 3kg satchel the buyer somehow changed it to standard delivery for $9 postage, when my settings dont allow buyers to edit totals is there a way they can still edit and change the postage?

Did the buyer buy multiple items? If the original listings were for $3 postage per item and the buyer has purchased 3 items then they would expect to only pay $9 postage. If you have changed it to $11.50 due to the increased weight then in checkout they can select to uncombine the invoice and save themselves the $2.50 by paying separately. If that is what's happened then I'd post them separately.