Interest?

So I get the visa bill several days ago. I don't often look at the bill as I do it all online. I check the balances and I pay it online. So I am just about to shred it when I notice there is interest charges. Say what?? So I investigate further. Seems I underpaid the previous month by 4 cents and that prompted a $2.56 interest charge. Crap, crap, crap. At least it was a small bill and so the charge is small, but one I wouldn't ever choose to make. Good thing I had already overpaid the bill this time so it covered the interest charge so there won't be one next month.