Not a must but ...
Not a must but whenever I have a big note to buy something small, I would always state, "Sumimasen, ookii no shika motte imasen.", which literally means that, "I'm sorry that I only have a big note."
For me, it is just a small way to show our gratitude for the pain/trouble we are causing. Small matter but it does maintain a good relationship no matter how short.
Andrew Soh Chih Jen