Sometimes when an order item is appended or existing order items are updated, a "Please Select Your Shipment Method" error occurs. I'm not exactly sure what causes this but it seems to be intermittent because sometimes I'm able to perform both of the operations without the error occur.
The problem seems to be that an extra empty ship group is added to the cart and takes the index position of first ship group. Because the empty ship group contains no shipmentMethodTypeId or carrierPartyId the error occurs.
Logic needs to exist to prevent the empty ship group that causes this issue from ever being added to the cart.