The only way to use a USB stick to take to the store is to first transfer the pictures to your computer. Then, you can copy them to the stick to take to the store.
However, may I suggest you loathe a look in the App Store and see if the place to which you want to take the pictures has an app? I've seen some of the pharmacy stores, and Walmart, have an app that you might could use. Then, you could do all the uploading from your iPad by using the app.
Maybe that'll help?
Marilyn