The end user who creates the Space will be the Space Owner. They can then designate you, the reseller or purchaser, as the Billing-Only Administrator. Note that this role does not grant you access to their projects or wireframes.
Check your email to find the link inviting you to join your end user’s Space. Once you have completed the steps to create your account and have accepted the invitation, you will be able to view their billing page. That is the section where you will select the plan they wish to subscribe to, and you will also enter your payment details.
And that’s it. You, as the Billing-Only Admin, will be able to change the plan as often as needed. Your end user will not be able to make changes to billing unless they know the credit card details you entered. They will, however, be able to cancel the subscription if they wish to.
Invoices will be generated and emailed to you as well as the Space Owner(s) every month or year, depending on the selected frequency.