The lines are LONG, and if you want to prepay for your plates and/or registration, here is the site to do it.
You will be asked for two things on the site. The first is the plate number "Numero de Placa". The second is the last 5 digit of the vehicle. If you look at your green registration card, on the right hand side, there is group of number under the heading "NIV". That is the numbers you want, and only use the LAST 5 numbers from the series.
Número de Placa: * Indispensable.
Número de Serie: * Indispensable (últimos 5 dígitos)
I did this last week, and it was a breeze, and I bypast all the people who didn't prepay.