Shipping method was wrong

Ok, 2 people now tried to buy a product using Media mail postage which is not correct. Possible suggestion; 2 ideas:

1. When we see the invoice; make the postage listed as a drop-down to select the correct postage option including cost and an updated invoice sent to the customer to pay the difference.
2. We are able to add a line item of the difference of the postage cost and email is sent to the customer to pay the difference.