Service fee in the sales order

Hello Experts ,

We have a scenario , where the sales order ( Ex: OR) will have a service fee line item in some of the orders. This is more like a design fee which will be added as one of the line items in the sales order . This item is not relevant for delivery .

The service fee item has been set up in the system with item category TAW . ( not delivery relevant ) The requirement from business is , they don't want this item ( design fee) to get invoiced until the whole order is invoiced. They can put a item level billing block , but again it's lot of manual intervention to place and remove the block .

Is there any smart way of handling this which will ensure that this item doesn't get billed in advance . Could anything be done at item category level or should it be a custom development . Please share your inputs on how this could be done with minimal system changes .

Note : This is not a service order, but rather a design fee added as part of normal sales order.

Thanks

Sri