Adding a gift/sample to an order from Amazon and track it in inventory (editing a store order)

How to add a gift/sample to an Amazon order thus include this gift item to the shipment and allocate it in your inventory?

Neither Amazon.ca nor ShipStation allow to modify orders.

Creating a duplicating manual order, copying all the order information line by line, adding the gift item, manually updating Amazon about the shipment and cancelling the original order in ShipStation is not acceptable.

Any other way, please?

And another use case with an order from a store - replace the same 4 individual items having one SKU with a box of 4 the same items having another SKU - how to do this?

Thanks!