If you didn't heard about SS (Silver Stripe) before now don't bid to the project.
Project: SS Billing Module similar to SS Ecommerce module, but with
billing and yearly and monthly invoicing, and to have extra "add on" option for client when ordering a product. The "add on" should be paid or free (based what admin is choosing when creating the product and adding the add on).
Further details will be provided via PM.