this is not a supported modification of power store.
the power store email receipt is generated by the webassist/WA_UniversalEmail/Templates/receipt.php and pp_receipt.php files.
you can add the necessary information to those files to include it in the emailed receipts.
if you need to also store it in the database when the order is placed, you would need to edit the ps4_oreders table to add the new column
and edit the webassist/plugins/shopping cart/store_cart.php
if you need assistance making this change, we can help you in a premiere support appointment