You have indicated that you are using the FREE version, which may be why you are not prompted to pay.  

Also, keep in mind that you do not pay to Netfile, you pay to use the software which is always prompted after you have completed your tax return, just before you Netfile.  

Once in your tax return, if you want to upgrade to a different version, you can click at the bottom left-hand side of your screen - Upgrade.