Simply set a maximum shipping amount to "Maximum shipping charged per order for national shipping/international shipping".
Please note: Ticking "Charge once per product for national shipping, even if more than one is purchased" will mean that if a customer buys the same product but buys 3 of that product the shipping is only charged once. But if they buy 3 of your different products they will be charged 3 x the shipping costs unless the "Maximum shipping charged per order for national shipping/international shipping" is set to a value.