ProtonMail is giving existing paid subscribers 5GB of extra storage space for free. The bonus is a way to thank them for their continued support. The freebie is permanent as long as you continue your subscription with the encrypted email platform. Free email accounts have limited functionality and only have 500MB storage, with their starter emails using 724KB. Free emails can be deleted to reclaim that storage, and the freebie has a value of 45 if paid a year up front. The cheapest paid Protonmail account is 5.00/mo for the Plus plan, which includes 5GB storage.
Source: https://www.bleepingcomputer.com/news/software/protonmail-thanks-paid-accounts-by-giving-5gb-extra-storage/