While we don’t currently offer official support for WooCommerce, you can create a custom implementation yourself. This would likely require you to embed the widget at the end of your current payment process. You can use it with WordPress. WooCommerce has a plugin available called Custom Thank You Pages that allows you to customize the Thank You page. When you customize the Thank You page, just add the shortcode for the ecard that you want the customer to be able to send.
That said we do offer an official Shopify App and we also offer integrated Stripe payment support to accept payments for eCards.