3v card.

I was been told about the 3v visa prepaid vouchers from a friend and I am after getting my one in the post but I don't get how it works. It tells me I have to go to the payzone shop (I know the shop that does it) and that I have to 'pay the value of the voucher and the price for each voucher' Then it says when I buy my 3v voucher I can go shoping but I dont get how much it is to buy it and to top it up.

These are the costs it says
(this is in a bar chart thing)

3v voucher value price
euro 20 free
euro 30 - euro 100 euro 2.50
euro 110 - euro 350 euro 5.00

Can anyone explain it to me further cause its confusing me

I know its a long shot thinking someone on here will no but just if you can please comment. thanks xxxxxxx