Plugin vendors do not manage licensing. 

Plugin vendors do not have any access to licensing. Atlassian is responsible for managing licenses.

Atlassian do not offer to extend 30-days trial period for Cloud add-ons. It is possible only for Server add-ons.

Refund first month of paid service

You only can ask for refund of 1st month of paid service. Please read about End of Evaluation (answer no 5.)