Indeed, the connection to the website is not secured, but the connection through the payment merchant is secure, because it uses HumbleBundle. I have purchased a ton on games from HumbleBundle and never had any issues. On the official website, use the bottom page to enter the data and I can assure you this is secure and encrypted. Alternatively, you can use Steam to buy the software and everyone knows Steam offers secure payment protection. I have also bought games through Steam and again everything was OK.
Note: Steam requires an account, because the purchase is digital and is tied to the account forever.