When merchants use Authorize Net gateway, there usually have 2 options for them for AUTH_CAPTURE and AUTH_ONLY.

For those merchants set to use AUTH_ONLY, this type of transaction creates an authorization with the card issuing bank that holds the funds until you wish to manually capture them.

To manually capture an Authorize Only transaction, please follow these steps:

1. Log into your Merchant Interface at https://account.authorize.net/

2. Click Unsettled Transactions in the main left side menu.

3. Click the transaction ID for the transaction that you wish to capture.

4. Click Capture. A pop-up window titled Capture Transaction appears.

5. If desired, adjust the Settlement/Capture Amount. Note that you may only adjust the amount to a lesser value. The transaction cannot be settled for more than the original authorized amount, but may be settled for less than that amount.

6. Click OK.

The transaction status changes to Captured/Pending Settlement and is queued to settle after your Transaction Cut-Off Time.

NOTE :
The AUTH_ONLY transaction type will not able to be revised more than the amount charged in Merchant Interface.
Using AUTH ONLY freezes the funds on a customer's credit card. So by freezing more then they requested not only do you limit their access to their own money, but you risk a chargeback because you froze more then you were allowed to. Only AUTH ONLY the amount you need.

For some merchants that want to work out shopping costs after transaction made, then you should charge the shipping cost separately using Authorize Net Virtual Terminal.

You can watch the video tutorial from Authorize Net about how to use Virtual Terminal to charge a credit card (locate video title "Virtual Terminal") :
https://www.authorize.net/videos/