What section of law are you referencing from. The only reference I'm aware of is that if you charge more than 5% of installment as a late fee, you will revoke other considerations. Specifically look to 658.49. The only other reference I know of is under usury section 687.03 and again limits to 5%. This part appears (without really disecting it) to apply only to retail contracts from sellers of goods...which would apply to banks dealing with retail contracts. This also appears to be limited to merchandise or goods for OTHER THAN personal, household or family purposes (which seems odd to me)
Are you looking at a different section?
_________________________
My opinion only. Not legal advice.
Say you'll haunt me - Stone Sour