One invoice for one sale order

Hi Experts

we have a requirement that sale order with multiple line should have only one invoice. But as of now invoice is generating for multiple line E.g SO with 4 lines out 2 is awaiting billing and remaining lines are awaiting shipping. if i ran Import Auto invoice it will create one invoice with 2 line.

we need header level invoicing It will not allow ANY lines to be invoiced until ALL lines are eligible for invoicing.

Can any one provide me solution.

Thanks