I am surprised you were treated so badly. When i was a day late (it was over a weekend) on a $23.00 balance on a visa bill, they billed me their late fee what, $25 or was it $40? and interest; some few dollars. I went ballistic too. I called them up, they saw I had paid and they forgave the extra debt.

Being pleasant and firm always helps. good luck. if not, i'd change banks.