You can sell Domain as is or related with Hosting Product. So in form area if you want to allow this product to create new orders as is, set SignUp Visible on. If this domain is just for Hosting propose, set SignUp visible off and Active Off.
Create Domain Prices
ZBilling allows you to set a price for a product for X days + Y months
Payment Type: Free, One Time payment and Recurrent.
Allow Signup: If this domain is available throw New Order Webpage.
Days: If recurrent, this field will be used to SUM next billing invoice date. Example: 15, invoice will be generated every 15 days.
Months: If recurrent, this field will be used to SUM the next billing invoice date. Example: 6, invoice will be generated every 6 Months.
TLD: .COM, .NET,…
If you want a domain .COM a recurring payment by 1 year or 2 years, add 2 price items with TLD .com with 12 and 24 in Month field.