WooCommerce Checkout Add-Ons 2.7.4

* Misc - Add SkyVerge help menu for support on WooCommerce.com connected sites
* Dev - Add filter `wc_checkout_add_ons_should_display` which allows merchants to override the displaying rules of an add-on

* Tweak - Add a customizer setting to allow to apply percentage adjustments on top of the total or subtotal

* Misc - Add support for WooCommerce 4.1