With Billdu online store modules, you can use our free modules to connect your online store with Billdu. Modules for OpenCart 2.x, OpenCart 1.5, Prestastop and WooCommerce can be found here. There is a brief guide in each file on how to set up the module for your invoices to be created or sent via Billdu.
Your API key and secret can be found in your Billdu account on the website under Settings - API.
We're sorry, it's currently not possible to connect with Shopify.
If you have any issues or feature requests concerning the general usage of the API please use the issue section in this Github repository https://github.com/billduapp/api_documentation