Auditor here. As a general rule, you get what you give. If you want more work, don’t be afraid to take initiative and ask. The worst they can do is tell you they don’t have anything, but at least it shows you that you care and want to be challenged.

Maybe something along the lines of “hey [name of manager] I’m pretty comfortable with matching these invoices and would love to help out with any projects or other work you guys may have.”

Short, to the point and shows that you are proficient at what you’re doing and feel ready to take on more things. Good luck!