To integrate API Key of Canada Post, and to optimize the website Flatsome WordPress, WooCommerce

I am at the last step for my website, done by using WordPress and WooCommerce. I want to integrate the API key from Canada Post (I have this API Key, and an account at Canada Post). Do I have to go by third parties, or this is possible to integrate that easily without a plugin? I don't know if this is feasable, but if this it, I will prefer that solution. Also, some little details that I want to fix, like when the shopping cart is empty, just not suggesting to go the Shop page, but to the previous page ( a little bit of coding)... and if you see some improvments to optimize the site, I will consider.