Hi Everyone

Could anyone recommend a daily commute backpack (3L area would be sufficient) that DOES NOT create shoulder pain?
I'm currently using a Camelbak and my shoulder's get so sore that I have to see a massage therapist regularly to get the "knots" out.
Even with 5 pounds of load, it's painful, and my commute is 20 miles both ways, about 1 hour ride, nothing major.

Forgot to add that I have a carbon bike so unfortunately racks are out of the question.

EDIT: I found the Ergon backpacks, but there smallest is a 12 litre.

Thanks!
Mike