When offering annual contracts w/ billing upfront, is it good to have a 3 or 6-month opt-out clause w/ a refund? or is it better to charge twice? at the beginning of each 6-month cycle & not touch refunds? or better to eliminate any opt-out options?

we eliminate opt-out clauses. we say a firm no to any refunds for licenses. If the clients want 3 month or 6 month billing the price goes up in a fashion where they would prefer to go with an upfront yearly payment. We offer refunds for unused data but that’s also prorated on a monthly basis instead of refund based on balances.