what you need is some simple and cheap way of doing. The only stand alone prepaid system I implemented used prepaid cards with magnetic strips. A magnetic card reader read the amount and then wiped the strip. However, the card reader was very expensive (approx. £500).
Is your meter connected to the internet, e.g. for transmission of water usage? If so your users could logon to a server and pay using their cried card. The meter, when the exsiting funds were running out, could contact the server using TCP to check for new funds.