There is a trend that new terms often begin as two words (e.g., “electronic mail”) evolve into a hyphenated form (e.g., “e-mail”), and then finally are accepted and commonly used as a single word (e.g., “email”). “Online”—yes, one word—has a similar story.
Stylebooks have evolved on matters of “e-mail,” “e-commerce,” “web-site,” and “on-line” as these terms have become a part of our everyday lives. Capitalization is lost, and hyphens are thrown out with yesterday’s floppy disks. Now, “email,” “ecommerce,” “website,” and “online” are the recommended forms of these words.
Unless you’re still printing your words on your dot matrix printer, I highly suggest you follow suit.