ubercart invoice generation to allow user to request funds withdrawal
If you are implementing the "service" as part of your site, you can attach the "services" to the user by assigning permissions to the "service" to a role, then create a content type, enable it as product and attach a "feature" to a product to enable the role for a limited amount of time (if this is your business model).

This would use CCK (to define the product) + Ubercart (for setting up the shop and attaching Product + feature to product) as well as Permissions (to your "service") and Roles (to apply to the permissions).

I hope this has been of some help!