CFE bill

I have registered on the CFE site and want to be able to print out my current invoice. The information request page asks for three pieces of information. They are:
1. RFC 2. SERIE 3. INVOICE
I have examined my previous bills and cannot find the term SERIE. What is it asking for?
In case CFE stops mailing bills it would be great to print it out.

Any help will be greatly appreciated.
TIA

If you have signed up for email they email a pdf invoice for you to print.

You are looking on the wrong page. You are now looking at the page for Factura which is something only used for taxes. I think what you are looking for is your normal bill which you should find here (when you are logged in)
https://app.cfe.mx/Aplicaciones/CCFE/Recibos/Consulta/Default.aspx
where you should be able to download your current bill.
If nothing shows up yet there, then you should still register your service number under the link "Agregar o eliminar número de servicio" There they will ask for a few things you will find on a previous bill.

I tried downloading the pdf a number of times only to get an error message...please try again later. Then one day a miracle...it just magically appeared as an email from CFE. Hang in there!