"Costs extra compared to the rest of the content".
No, it costs the same. If you send 10 gb of emails, it costs the same as 10gb of netflix in bandwidth and pipe use. The difference is before the customers did not use the ISP at all, they just sent emails.
Now the customer wishes to use the service they are paying to the full. This creates a problem, as the ISP was playing the "spread the costs over low users" game. Which leaves them in a sticky situation when the real figures show they cannot server the customers with the product they were charging for...
(This sometimes happens with airline companies. They sell 310 seats on a 300 seat plane in hope 10 people don't show up because of reasons, like work turning down holiday or being ill with a cold etc)