Here's a canned response. First, log into Show Themes: https://www.showthemes.com/wp-admin/. Select Woo Commerce > Orders > Search for the user > and note the Date of the order i.e. September 6, 2018 (you will need that date for the response). Use this tag: False Expired Notice.

Thanks for reaching out to us. You are absolutely correct that your membership is active and that you renewed in September 2017. It appears that your email address was incorrectly identified as having an expired account. Please accept our sincere apologies.

We so appreciate and thank you for your feedback. It's members like you who help to keep us on our toes.

If an active member received the email in error, and wants the discount, respond with this canned message:

There's no need to renew now. Your membership will continue to be active until September 2020. In September, you will be sent an automatic invoice for renewal. Since the promotion will not be in effect at that time, kindly send in a support ticket after your payment processes successfully (and reference this ticket). We will be happy to issue a partial refund to you (and honor this discount at that time).

Thanks again for your understanding.