How to determine customer maintenance expiry

We are developing a plugin for JIRA Server. We plan on offering vendor support for the plugin, and the plugin will be a paid-via-Atlassian add-on listed on the Atlassian Marketplace. Support would be via our existing support website.
However, we would only offer support for customers inside the maintenance period (similar to the Atlassian Support model).

How can we determine that our customer’s plugin license is still within the validity period for maintenance, from our external support website? Is there an API we can call with their SEN number for instance?

Sounds like you’ll want to use the Marketplace API.