Your client does not need an account
Send a link or the PDF. They open the invoice and download it. They do not sign up for Wire Invoice.
Two ways to send it
Download the A4 PDF and attach it, or send the link. The link opens the same invoice. There is no login on their side.
What they can see
The commercial invoice: who billed them, the lines, the total, the currency, and the bank details. Not your dashboard, not your other clients.
The link is the invoice
If you change the invoice after you share it, they see the new version at that link. If the amount changed and they may already have approved the old one, issue a new number instead.