Scenario 

Example:

In Standard format unit price will display blank, If user did not insert any of unit price. Maybe this 2 item are free of charge.

But it also can be customize as above picture, to know how to customize please check solution:

Solution 

Step 1 – Open a sample invoice à Click Task.

Step 2 – Click Design.

Step 3 – Once Click Design , report  format list will appear, double click any of the invoice format.

Once double click to invoice format, please give a format display name and format code.

Any name and any code.

Step 4 – once go in to the format, Click on unit price, right click the unit price à Click Add calculated field.

Step 5 – once click Add Calculated field à a ‘calculated field 1’ will appear à Right Click it and Click edit calculated field 

Step 6 – insert a name, and expression please insert as below:

Iif(IsNullOrEmpty([Unit Price])||[Unit Price]=0,'Free of charge' ,[Unit Price] )

Click OK.

Then Click Save.

Last Check the result.