Do you have the activation codes in the cart when the user is checking out? If the values are available to you before the user checks out then you can get them in the cart with these input fields on the confirm page:
<input type="hidden" name="on0_<?php echo $eCart1_Index+1; ?>" id="on0_<?php echo $eCart1_Index+1; ?>" value="" />
<input type="hidden" name="os0_<?php echo $eCart1_Index+1; ?>" id="os0_<?php echo $eCart1_Index+1; ?>" value="" />
<input type="hidden" name="on1_<?php echo $eCart1_Index+1; ?>" id="on1_<?php echo $eCart1_Index+1; ?>" value="" />
<input type="hidden" name="os1_<?php echo $eCart1_Index+1; ?>" id="os1_<?php echo $eCart1_Index+1; ?>" value="" />
These fields constitute a name value pair, so for the first one you can set it's value to 'access code 1' and for the os0 you would set the value to the access code 1 value. You could do the same thing for the next access code, set the value of on1 to be 'access code 2' and for os1 you would set the value to the second access code. Then on your IPN page you will look for these fields to set the values for your update. You should be able to make a test post with these values to see how your page reacts to them.