There are 2 methods for having another Pobox customer receive your bill.
If you would like another Pobox customer to pay for your account, invoiced separately from their own account, just go to your authorizations page, and enter their email address as your billing contact. They will be sent an email asking them to confirm that the request to pay for your account. Once they approve the request, your bills will be sent to them in the future.
If they would like the bill for your account to be included with their account on a single bill (instead of both accounts being billed separately), just send an email to Customer Support asking them to merge the bills for the two accounts.