How to add a custom fields on invoice?


I've an ugent problem:

I need add a custom fields on customer invoice.

I added a custom field (label: “Codice Fiscale”) for registration on “profile fields” in admin panel, but It does not appear on invoice… How can I do, please?

On this forum I read that it should be modified tpl file in my skin:

" {include file=“profiles/profiles_extra_fields.tpl” fields=$profile_fields.B}"

…but I'm not abre to do it.

Please, somebody can help me?