Changing Price level on invoice

Is there a way to add a dropdown field on an invoice to change the price level used for the entire invoice? Our price levels are based on total quantity ordered, not just the quantity of one item. Price levels are set in the customer record for standard orders, but if a customer orders over the next price level, they get that discounted price. When invoicing, each line item must then be edited to change the price level. Is there a way to chose the price level for the invoice and have it apply to the whole invoice for this