A strict reading of the §1005.6 rules starts your counting of days with the day the statement was provided that included the March 17 transaction. For example, assuming that you determine that all four transactions were unauthorized, and assuming the statement was sent on March 31, count out 60 days from then (May 30). The member gets reimbursed for the March transaction, but is responsible for the August transactions.
If you charge back the August transactions, you will also have to credit the member for those chargebacks, but you'd be doing it under the Mastercard or Visa rules, not Regulation E. Then, if the merchant successfully represents the transactions, you can take the funds back from the member (because you're not working with Reg E now).
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8