Hi,
You have to edit this PHP file to add the "additional custom notes"
/templates/YOUR_TEMPLATE/html/com_virtuemart/cart/default.php
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but if you don't have enough experience on PHP, i don't think that you will achieve that. Because "Customer Notes" is being generated via "Shopper Fields" (VM Backend >> Configuration >> Shopper Fields). You need to edit some core VM files as well, which is beyond our "free-support"
Regards.