I find it best to have an email created specifically for the purpose of receiving the code.
Also make sure that you are registered with Baen using a DIFFERENT email address.
When you go through the check-out process make sure you put the correct email address in step 5 (NOT the registered one).
The instructions in this link helped me when I first went through the process - had them printed beside me and "ticked the boxes" as I went just to make sure - really didnt want to muck it up.
Link to helpful instructions
Its step 10 in the instructions when you use the email created to hold the code.
When someone PM's you for the code, you can reply with the code and/or forward the email on to them.
Hope this helps