Some post offices WILL require ID for everyone receiving mail there. However, most of them won't. You just need to fill out your real information on the first page, then in the section asking who else will be receiving mail there, put your alias. When I got my PO box, I simply told the cashier that I was a model and would be getting packages under the alias. She was cool with it, just told me where to put the name. If they won't let you do that for whatever reason, go to a different post office. Or come back on a different day (to hopefully get a worker who's more lax with the rules).