How Can I extend evaluation period

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.)