Can Verizon's international calling plans be added to just one line in a multi-line account?

I have a Verizon Wireless account with four lines (four phone numbers) together on the account.  It is the type of account which Vz calls Small/Medium/Large, etc.  I understand that for $5 per month per line, I can extend my unlimited calling to Canada and Mexico (when placed from the United States).

What is not completely clear is this:

I want to do this for just one line.  From only one of the phones on my account do I need this capability.

If I sign up, will my bill increase by 5 dollars each month?  Or will it increase by 20 dollars, because I have four lines and will be forced to pay $5 per month *per line* (and the service extended to all four lines)?