I am getting an error when I try to bill my client. It says that I must have an e-mail address to which to send it, but in her personal info section I do have three e-mail addresses in the "Invoice...

You are right, you do have emails listed for this client. I noticed that in the "Invoice Email Address" field you have 3 email addresses listed and they are separated by a comma (","). In fact, if you have multiple email addresses listed, you just need to separate them with a semicolon (";"), that's all.

The system saw the commas and didn't know how to read that character. Change the commas to semicolons, make sure you click "Save", and you'll be all set.