you can open other people's post as long as there is no intention to act to that other person's detriment.
But do you have to ?
I'm am still plugging away at the situation the OP stated, where all negotiations have been done with person "X", but when the first bill arrives, it is addressed to person "Y".
What would happen in the event that person "Y" (1) refused to pay (2) was not available (i.e. working abroad) and person "X" (for whatever reason) did not open any letters addressed to "Y", and the bill went unpaid ?
If the above situation came before a court (although I suspect any company would not let it get that far, in case there was a ruling they didn't like) where a company was suing for payment, and the defence was they had misaddressed their invoices, what would the courts view be ?
(1) That regardless of addressing, someone in the household should have opened the invoice to pay it ?
or
(2) The lack of payment was entirely due to the companys misaddressing, and the court will not order person "Y" to pay for person "X".
Bearing in mind the ongoing battle (which has been fought since I can remember) to ensure both halves in a partnership have the right to financial autonomy.