Hi,
Thanks for the interest in our plugins.
Can I or the customer buy one license for each add on and use it first for the development page and later on the live page?
Yes you can buy the one license for each plugin and then use that on the dev site, then you can transfer the license to the live site.
The license is only for updates so it is not required for the plugins to work.
We also have this guide about dev and live sites:
https://theeventscalendar.com/knowledgebase/using-one-license-for-your-live-and-dev-sites/
Let me know if you have any follow up questions.
Thanks