WooCommerce Points and Rewards 1.8.0

* Fix - Unable to apply points from the checkout page to an order with a subscription product.

* Fix - Prevent fatal error when viewing coupons in admin with Subscriptions.
* Fix - Update reward messages when using a regular coupon.
* Fix - Only apply points to previous orders which have been paid.
* Tweak - Use standard format for help tips in settings.
* Tweak - Remove legacy code for wc_points_balance.

1.6.37
* Fix - Update Min Points Discount tooltip to more accurately describe the setting.