before I found your amazing product, I have been using Shopify's "Buy Now Button" and their coupon code function has two very useful details which you could also add to your Membership2 coupon codes, in my opinion, to provide users with a better experience:
1) limit the number of times the SAME user can redeem a coupon code (this is where abuse can take off sometimes) - right now you only allow me to limit the total number of redemptions, which is not the same.
2) specify RELATIVE discounts (e.g. percentages, like 50%), not only ABSOLUTE values (1 EUR), as I can do now. This is something your - otherwise great - product lacks now.
Let me know if you need more information from me.