I need a FASTER way to change shipped from addresses WITHOUT having to BUY the label FIRST!!!

I keep my inventory in my SUV. I could be shipping from 3 different towns a day. Your system of changing "ship from" addresses STINKS!!! I'M AT MY SONS PLACE IN THE A.M. AND MY DAUGHTERS PLACE IN THE P.M. then back again. USPO will not send if wrong address is printed.