I went through this for V14 (still am).
Basically they need an account, one way or another in my exploration.   On a web order or left shopping cart situation they don't necessarily see the account or even use a password, but a partner account is created when passed from being a "Public" customer to a named user.

If you want to share a quote built on the back end, the complication seems to show itself around "payment acquirers" if you send a "payment link".  If logged in, Payment Acquirers are available.  If not, at least in my exploration, they are not.

To rectify this it is best to generate a quote.  Send a Portal Invite, ask them to sign up, then send the payment link.  Perhaps you could put the two together on one email but default it is in 2 emails.

IMHO this is a shortcoming of the process and it is not well documented.  Using backend generated quotes is important.