We had 2Checkout account closed too (2checkout closed 90% of hosting companies accounts). You can open a ticket on 2checkout's help desk and ask them to refund desired transactions. Just give them in a ticket the list of transactions you wish to be refunded. If you have sufficient funds in your account, they will refund it to your clients. They locked your funds for 3 months exactly for this reason - chargebacks and refunds. I would suggest you to avoid asking clients to chargeback. Chargeback fees are $50 per chargeback on 2Checkout. I'm preety sure you don't want to pay $50 to every client who request chargeback. Good luck with your business!