Billing interface populate unit price and calculate total amount based on quantity and unit price

We build our UK customer with same invoice line with hours different each month. We use billing charge ID and identifier for billing rate which will be the unit price for invoice line. When I enter invoice on-line, the system calculate gross extended amount from quantity(hours) and unit price (default from Identifier). The billing upload should function as the on-line to pull the unit price from charge id table and calculate the gross extended amount field.