You can easily cancel/void any shipments that you do in Momentum as long as the label isn't in use (package already picked up by the carrier).
FOR SINGLE SHIPMENTS, DO THE FOLLOWING
1. On the main screen, click on track
2. Click on the line item you want to void
3. Find the button to cancel and answer yes to cancel.
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FOR BATCH SHIPMENTS, DO THE FOLLOWING
1. On the main screen, click on Track and then click on the BATCH sub tab
2. From here, you can do a cancel the entire batch or you can click into any batch and cancel individual addresses within the batch.