send.zip (the online service) and send.zip (the web app you have loaded right now) are two different programs. Encryption is done in the latter, in your browser, without connecting to the internet, your file is loaded into ram, encrypted using AES, then uploaded without the key. This also means if you lose your key I can not help you recover your files. ))):
send.zip uses cookies when you log in to grant access to additional features like faster uploads and downloads. Cookies are also used for most functionality on send.zip even if you aren't logged in to help prove that you're human, this is effectively an account without signing in.
No
All files are encrypted by default, and need to be decrypted before they are saved. A file is fully downloaded to memory then run through the decryption step, the decrypted file is then downloaded from your browsers memory, as opposed to the internet. As far as I know there is no way around this given the current state of the web.
send.zip is a file transfer service, not a file storage service. Storage is far more expensive to give away for free, or even sell. I may add a paid tier for file storage when send.zip has scaled up enough to make this viable.
The average user really does not spend that much bandwidth, so a small amount of paid users can effectively subsidize the site for everyone else. Even if there are no paid users, I have a large unused bandwidth allowance from my other sites which only use at most 1% of my bandwidth, leaving the other 99% for send.zip.
I plan to keep the core file sending service free forever (with no bullshit) for both practical and moral reasons. It is more apparent every day that the most important resource of the 21st century is attention, and so much of it is spent on meaninglessly complex tasks like finding a way to send a 9mb file to your friends (while streaming in 720p to them for the last 10 hours).
Right now, all backend API calls are logged, and files you upload have the file name and last modified date stored in plaintext for as long as the file is uploaded.