If you are using BigCommerce you have one of the best e-commerce platforms.
Now you can have a free WhatsApp Widget for your website.
Create your account for free and personalise your Widget.
Then you need to copy and paste the code in your Website.
How to add a WhatsApp Widget in your BigCommerce Website
If you need to edit your header section you need to go to > Templates > components > common >
There you can find your header template and edit the file.
You will be able to copy the script you just generate with your free account.