To set up different types of top-up vouchers go to:
Customers -> General rates -> Top-ups/balance charges
There you can create the types of top-up vouchers you want so the customers can buy them.
If you want to add new types press on "New", if you want to modify already existing ones press onand if you want to remove a type press on.
When you want to add new types, you'll see the following window:
The starting and ending date are for offers limited in time, so you can tell the system when the offer would be available. If you don't write anything here it will be available until you delete it.
The purchase amount will be the amount the customer is paying, and the balance to recharge will be the amount that will be added to the customer's account.
If you want to top up a client's account you can check out how to do it here: Top up customer's account