What I'd really like to know is:

"Down-payments to the amount of 350 EUR for a NeoN board or 480 EUR for complete Neo900 device are needed by XXXX-XX-XX to guarantee one will be built for you to purchase later. The sum of your prior donations (before interest) will count towards this.

"Current pricing estimates are shown at http://neo900.org/estimate and we hope to start shipping in QX XXXX. To claim your device, you will have up to X months from that date to pay the final purchase price, less your down payment (and [non-compound] interest applied to donations that were received before XXXX-XX-XX at a rate of 2% per month)."

p.s. joerg: I made a down-payment of 300 EUR last month because that's what was requested at the time, in order to guarantee a NeoN board; you're saying this has increased and a second payment is needed now?

p.p.s. for some future project or re-run, a monthly subscription sounds like something that should be accommodated if at all possible (including better automation for handling such payments). It could make for more steady project funding for R&D, and for risk purchases at intervals. It would help to more accurately gauge interest and estimate the number of devices to build, because you could see each month as subscriptions are created/cancelled. And I agree with endsormeans it makes it easier to budget and cover the final purchase price as a potential buyer.