If you need to cancel a shipment that is considered shipped by MyFulfillment, this is how to do it.
First, go to sales -> shipments and click on red trashcan of the shipment you want to delete.
A popup will appear and you just need to validate your input.
After that the shipment will be deleted and will no longer appear on that page.
As soon as the shipment is deleted, the order is going to be set to "processing" and a stock movement will be created to put the order's items back into the stock.
Then you can prepare the order from the "order preparation" page.
Delete the Shipments in mass:
You can delete the shipments in bulk too.
Choose the shipments >> From the drop-down select the "Delete" option >> Click on submit >> Confirm from the pop-up as "Ok".
This will make sure the shipments are deleted and the order will set to the "processing state" and a stock movement will be created to put the order's items back into the stock.