Robotnik

I did, but I want the email sent to the customer, not to me. Basically, like an additional invoice email containing additional info only when they buy certain products

No, both URL’s are just using plain Http.

I dont see any plugins like that, plugins I have are:
amr shortcode any widget
Awesome Surveys
Custom User Registration Form Builder Silver
Exclude Pages from Navigation
FuturePay for WooCommerce
Media Library Custom Fields
Profile Builder
Our Team Showcase
Quick Page/Post Redirect Plugin
Simple Page Ordering
Simple Social Buttons
WooCommerce
WooCommerce AJAX Products Filter
WooCommerce Product Add-ons
WooThemes Helper
Wordpress File Upload
WP Config File Editor
WP Gallery Custom Links
Or is it possible for disabled plugins to affect this?

Nevermind, I think I solved it. Restarting the apache server using /etc/init.d/apache2 seemed to solve the problem