Login to authorize.net
Then select "Fraud Detection Suite" from the left side column. (Highlighted in Red)
Click on the word "Amount Filter" (Highlighted in Red)
You can change the Upper Limit and lower limit to be notified about transactions outside of these limits. the Lower Limit will notify you if a transaction is run under this limit, and the upper limit will notify you if a transaction is run above this limit.
There are four filter actions.
Process as normal and report filter(s) triggered. - This setting will only notify you that a transaction was run outside of these limits.
Authorize and hole for Review - This will put a hold on the customers card, but will not capture the funds until you manually go in and review the transactions and capture it manually.
Do Not authorize, but hold for review. - This will not authorize the card, but will hold the transaction for review so you can manually authorize and capture the sale if it is a legitimate charge.
Decline the transaction - This will automatically decline all transactions that are outside of the limits set.
You can also disable the filter completely by unchecking the check box on the "Enable Filter" checkbox.
Then click Save to save the changes.