While you are not obligated to do so, if buyer has not paid yet, you can click on Send Invoice and add an additional service and cost for them to choose.
If already paid for, while you can not change the amount buyer paid, you can switch to a different carrier since the buyer requested it. Also per their request, if you want and the amount is more you can send them a PayPal invoice for the difference, or if amount is less you can send them a partial refund.
