Please try to understad i mean
If i sale a door to one customer thats size (72"36")in inch means 18 feet rate 100/feet than amount is 1800
If the same door i want to sale other customer rate 105/feet then i shuld calculate its amount by calculator then i will enter in rate column
If unit column is available in invoice then door size is continue 18 feet and i need only change in rate figures like 100 and 105 and so that
18feetqty1rate100/feet= amount 1800
18feetqty1*rate105/feet= amount 1890
only change in rate can create invoice easily