This article shares some possible use cases your app could support, as well as a sample flow that could support each use case. This can be a helpful jumping off point as you plan your app's implementation.
You could set up a site for a customer in your agency's team account. This way, the customer doesn't have access to the site until you've finished the setup and testing. Then, transfer the site to the customer's Wix account and assign a paid service to it. Note that you can't transfer sites with paid services.
To set up the site:
You could create a package of paid Wix services and assign the services to the relevant sites. When the customer realizes that want to use one of the services in a different site, you can re-assign it.
To re-assign a product instance:
productInstanceId
in the Unassign Product Instance From Site call.