I work for my clients. Can I test BYOB for free?

Yes, BYOB is a partner-friendly app. You can use the Free plan for development and testing purposes. 

Development Stores

Alternatively, you can change your plan to a paid plan if you're testing with a development store created by Shopify Partners. A test charge will be issued to you. When your development store switches to a paid Shopify plan, BYOB will switch to the free plan automatically. We'll send an email asking the store owner to upgrade. 

Sandbox Stores

If you're using a sandbox store created in your Plus account, these stores cannot be recognized because the plan name is "Shopify Plus". There is no way to accept a test charge like the development stores above.