Nanwa, I am sure we still charge a late fee on interest only loans. As long as your contract allows for it I don't think there are any rules against it.
As for how short, I think that is up to your bank. If you allow short payments of $10 and still advance the due date, as long as you do it across the board it shouldn't be a problem. Secondary market would obviously have their own limits but I don't know of a limit on the State law end of it.
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