The font handles the translation logic for you. You do not need to manually type +A to get a lowercase a . You simply type the text you want, and the font generates the correct sequence of wide and narrow bars so that the scanner reads it as the exact text you typed.
The most common support call for any Code 39 font (standard or extended) is: "My barcode won't scan!" 3 of 9 extended barcode font
Here’s a concise review of the (often called Code 39 Extended), based on common user experiences. The font handles the translation logic for you