Neha
Pre-shipment invoice is known as Pro-forma Invoice in SAP. You can use billing document type F5 (Pro-forma invoice for Order), created with reference to your sales order (process VA01:OR - VF01:F5). No accounting document is generated for this type of invoice.
Post-shipment invoice is known as Delivery Related Billing in SAP. You can use billing document type F2 (delivery related billing), created with reference to your delivery document (process VA01:OR - VL01N:LF - PGI - VF01:F2).
Regards, JP