If you are using WooCommerce on your WordPress website you may want to make a direct link that lets people add a product to their cart. This is one way to do that without using a plugin.
https://yourdomain.com/checkout/?add-to-cart=345
You replace the https and yourdomain.com with your http or https and the website domain and the 345 with the product id. You can read this post to find out how to find the product id of a WooCommerce product on your WordPress website on there.