Note:   By doing this you lose the ability to (a) check for fraudulently placed orders; and (b) risk reserving inventory (maybe even shipping it) without checking that the order was actually placed with an intention to pay.  We recommend you still review orders to eliminate the chances of fraud.

When you hover over the CONFIRM button on a Sales Quotation in Developer Mode, you can see that is calls a Python method called action_confirm

First I would make a Server Action that calls this method - and click the CREATE CONTEXTUAL ACTION button to add it to the Action Menu (shown when you select multiple Quotations)

This gives you a way to confirm multiple quotations at once, but you can also completely automate this process by creating an Automated Action that listens for a state change for orders coming only from the website:

Make sure to filter only for orders with Website is set and Payment Ref. is set and apply only when Status changes to "sent" which is what happens when the website User checks out their shopping cart.

Is it also possible to create an invoice at the same step?