I've seen a couple of apps which seem to enable merchants to accept payments from customers off platform (i.e. not on Shopify) and then create orders via the Orders API.
1. Is that allowed?
2. Does the merchant or app developer get charged any fees for orders created via the Orders API?
ReCharge asks merchants who have installed their app to connect their Stripe account. Customers go to the merchant's online store, click to buy a subscription product, go to ReCharge's checkout form (not Shopify) which then processes the payment and then creates an order via the Orders API.
The Facebook app does something similar too since it allows customers to checkout on Facebook and then pays out to the merchant's bank account.