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If I cancel an order, will the payment transaction be voided or refund automatically in Authorize.NET?
If I do “Credit Memo” in magento admin console, will the refund information go to Authorize.NET automatially?
No ... the best way to handle this situation is to take an authorization at the point of order. If you need to cancel the order before it’s shipped you can release the funds manually. Regarding refunds again there is no integration with the PSPs, you need to manually issue a refund. A credit note is just that ... it stays on the users account and can be redeemed against later purchases.