We'll send notification email to owner of Shifter account before exceeding storage limit and data transfer limit of plan.
Check the latest statistics on Shifter Dashboard and upgrade plan to keep your site alive.
Fore more detail check this document:
What happens if storage or data transfer exceeds plan's limit?
Shifter supports WebP media format under Shifter Media CDN enabled environment. Just enable this feature, Shifter converts media to web format without changing original.