Hyde here from Shopify.
If you're looking for invoices from Shopify for subscriptions and apps, then you will find that under Settings > Billing.
If you're looking to send invoices to your customers, you can do so manually using our draft invoice feature. For automatic options, you would need to enlist the help of one of the invoicing apps available on our app store here.
I hope that helps!
All the best, Hyde.