Updating multiple iPhones

Hey all 🙂

I work in an Apple reseller. We often do iOS updates for customers on their iPhones or iPads. Everytime I do an update for a customer, iTunes downloads the software update. I was wondering if it was possible to downlaod it once, and always use that file (until a newer iOS is released). Just seems an inconvenience that I have to redownload the software every single time. Any advice appreciated 🙂