envios "prepagos"

hola, soy nuevo en esto, quisiera saber si hay alguna forma de configurar para que los clientes compren envios, ejemplo.

1000 envios a $5, y si a los 2 dias quieren comprar 1000 envios mas a $5, puedan sumarse los envios. y que tenga 2000 envios.
sin planes mensuales ni nada similar.

y otra consulta, como genero un link para que se registren en el sistema? y darles una cuenta gratuita (grupo ya creado) que tengo?
Google translator reads -

"Hello , I'm new at this , I wonder if there is any way to set up for customers to buy fresh , example .
1000 shipments at $ 5 , and if after 2 days want to buy 1000 shipments more than $ 5 , can join shipments . and has 2000 shipments.
no monthly or anything similar plans.
and another query , as generated a link to register in the system? and give them a free account (group already created ) I have?"

Translator gives me a hard time understanding your query ;) . But from what I understand, both are possible.

1. Go to Customers > Groups > Sending and set "Time value" to "-1". This group will have unlimited validity from now on.
2. You should enable customer registration feature for this. Go to Settings > Customers > Registration and flip "Enabled" to "Yes". To have the newly registered customers land on a free/any group, select that group from "Default group" drop down list.

Hope this helps, if it doesn't, someone who knows Español would help ;)
pero si un cliente compro 1000 envios, utilizo 50, le quedan 950.
quiere comprar 1000 mas, como hago para sumarle y que le quede 1950?
Ah got it now. This is something not possible at the moment. If a customer purchases two plans, the latest one applies.

Will leave to @twisted1919 to see if there's a work around available for this.