I am designing a Wix site for a client, how can he pay for the premium plan?
It is preferable for your client to manage his or her payments from inside his or her account. We recommend that you create the site in full before asking your client to upgrade it.
You can either create the site inside your client's account and let him or her upgrade it when it's ready. Or you can create the site in your own account and transfer it to your client once you have finished.