In extreemly simplistic terms, you must have understated either the finance charge or the APR (or both) to have to reimburse. You may have to refund (reimburse) finance charges already collected and may have to alter future payments (read that as reduce) to bring the APR or finance charges down to whatever you understated.
Reimbursement is a very complicated area. Unless you have done several, I would not try it without professional assistance, probably from a lawyer. I believe you are in CA. Give me a call and I can give you some names.
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Al Miller, CRCM
Fremont Bank (CA)
(510) 790-5825
(510) 505-5211 FAX
Opinions expressed are my own and not necessarily shared by my employer.