Does ZOOcart generate invoices?
It does not and it is not planned. Alternatively, and in some countries, the Order print version is a valid document which the user can access under the Print button in the Orders view.
Is it possible to specify the gross price instead of the net price?
Currently you can only specify the net price with the option to display the item price with tax (gross price) or price without tax (net price).
How can a Subscription be renewed from the frontend?
On the frontend Subscription View, the Renew button will be displayed after a Subscription is ended (and if this product still exists). After clicking on the Renew button the product will be added to the Cart and the user redirected to the Checkout.