Hi there!
Thanks for your input with this - I've added your suggestions as new features to consider when updating the PO system.
For what it's worth, even after you send an invoice to your buyer, you can update with additional charges (like a shipping charge) and add a note to resend to your buyer - as long as they haven't paid, you can edit the prices as necessary. That way, if you find an additional shipping charge is needed, you can update the PO and send a note so that everything is all together in the invoice and the pricing is correct!
If you think of anything else you think can help improve Etsy Wholesale, please let us know! We love hearing feedback like this from the community.
Thanks so much!