Firstly, thank you very much for supporting development by purchasing a copy of elementary OS. It's our primary source of income and the vast majority of people choose to never pay anything. So we really appreciate your initial payment.
If you ever need to re-download elementary OS, you can always click the "custom" button and type in "0". This will completely bypass the payment dialog.
However, I do please ask that for new versions you consider a small payment. Without continued funding, our development pace and infrastructure will be impacted.