The surest way to cancel an FBA multichannel fulfillment order is directly in your Seller Central account. Here's a guide from Amazon:
If you would like to cancel a Multi-Channel Fulfillment order, you can do so while the status of the order is "Planning." Orders move from "Planning" to "Shipping" quickly, so you may only have about 15 minutes to cancel the order. Follow these steps:
Locate the order under Manage Orders.
Click the Order ID link to view the order details.
Under Order Details for Multi-Channel Fulfillment Order, click "Cancel this Order.