While your question specifically asks for "using sites like way2sms, etc", an alternative is to use the email to SMS feature that all carriers seem to provide. So you'd just use JavaMail and fill in the information accordingly. Downside is you have to have different information for each carrier. I've done this before and it works very well.
For example from AT&T:
http://www.att.com/esupport/article.jsp?sid=KB63037&cv=820#fbid=gFn39JcTTp6
To send a text, picture, or video message to an AT&T wireless device from your email:
- Text message: Address your email to the recipient's 10-digit wireless
phone number, followed by @txt.att.net, for example,
5551234567@txt.att.net.
- Picture or video message: Address your email
to the recipient's 10-digit wireless phone number, followed by
@mms.att.net, for example, 5551234567@mms.att.net.
- Don't include dashes or spaces in between the numbers.
JavaMail API:
https://java.net/projects/javamail/pages/Home