Good afternoon!

I am happy to provide some information here. You can cancel or adjust shipments after they have been confirmed, but there are some stipulations what and when you can do that. Here's some info from Seller Central:

"Changing shipment contents after shipment creation

We understand that due to normal business events, you might need to change the contents for a shipment after shipment creation. You can do so by clicking View or edit contents in the shipment details box after confirming a shipment.

If your shipment contains multiple boxes, and you chose not to provide box content information or used 2D barcodes to provide box content information, then you can edit the number of units in the shipment through the All SKUs tab. For other methods of providing box content information, you can edit the number of units through the All boxes tab.

Important: Before the shipment arrives at the fulfillment center, you can edit the number of units in a shipment by up to 5% above or below the original amount up to six units. If you need to make changes above this limit, you must cancel the shipment and create a new one.

For small parcel shipments that use an Amazon partnered carrier, you have 24 hours to edit the box weight or dimensions, or add or remove boxes. The edits could result in a change to the estimated shipping fees. After you submit and confirm the edits, you must print and apply the box labels to all boxes."

I believe the italicized portion answers your original question. The link I provided also has an FAQ at the bottom, so I recommend reading through it entirely.

- Bryce