This page covers questions about BTCPay integrations.
At this time, BTCPay does not have a Shopify integration.
Yes, you can. While various e-commerce CMS use integrations, you can use BTCPay even if you're not a merchant. For more information about use-cases, see this page
Order status depends on a merchant's business model. To better understand BTCPay order(invoice) status read this document. There's no generally the best way to configure them without trial and error and seeing what works for your business. See this discussion for more information on order statuses.
If you wish to send an e-mail after a certain status to the customer, you can configure your BTCPay WooCommerce plugin. This is recommended only if you know what you're doing. Follow this community guide.