You can start with a single domain and later upgrade to multiple domains. You will only need to cover the difference in the price.
Let me know if you have any other questions.
I would like to let you know that the One Page Checkout is now styled for the Smart Theme. Please have in mind that the styling is not included in the downloadable file of the plugin. I will send you the necessary files by email if you decide to purchase the plugin.
The plugin is styled for the Motion and Allure Theme. We are planing to style it for the Smart Theme as well in the next week. I will let you know when the styling is available.