Is there a minimum requirement on the amount that is to be refunded to a participant who has terminated? e.g. A participant has overpaid by $1.32 on their premium for the month of May. Typically that amount would be applied to June's premium, but instead the participant decided to cancel the coverage as of May 31. Is there a minimum amount that we are required to return to the participant (e.g. anything under $25 doesn't have to be returned) or are all overpayments required to be refunded?

I would personally think that any amount would need to be refunded, otherwise you are collecting more than the 102% that you are allowed under law (assuming that you are charging the full 102%).