ReadyToShip doesn't currently support splitting an order into separate orders or partial fulfilment of single orders.

If you use Shopify, and items in that order are fulfilled from different inventory locations, then ReadyToShip will respect those locations when creating a fulfilment (but ReadyToShip can only fulfil all items in an order at once)

If you need to ship or fulfil line items in an order at different times, you can process orders like this manually in your carrier account, or you can split orders in your webstore/marketplace account. Those orders will then import separately into ReadyToShip.

If you process orders outside of ReadyToShip, they may turn red to alert you that changes have been made.