When you open a top-up, the page shows an address and an exact amount of crypto, and a clock counting down from thirty minutes. People ask why the amount cannot simply float with the market, why thirty rather than ten or sixty, and what happens to a payment that lands after the clock hits zero. The answers are about who carries the risk of a moving price, and we would rather explain the trade than hide it.
The problem a lock solves
Your balance is in dollars; you pay in a coin whose dollar price moves every second. Somebody has to decide which price applies: the one when you saw the page, the one when you pressed send, or the one when the network confirmed it. Those can differ by a percent or two on a normal day and by ten on a bad one. Without a rule, either you overpay or we under-receive, and neither of us can plan for it.
The rule we chose is the simplest one to reason about: the price you see is the price you get, for a fixed window. The amount on the page is computed when the address is created, from the mid-market rate at that second, and stays valid for thirty minutes. If you send exactly that amount within the window, the full dollar value lands in your balance regardless of what the market did in between. The risk of the price moving during those thirty minutes is ours.
Why thirty minutes
The window has to be long enough to actually pay, and short enough that the price risk stays small. Paying takes real time: opening a wallet, scanning a code, maybe withdrawing from an exchange, then waiting for the network. Bitcoin confirms in ten minutes on a good day and much longer when fees spike; Monero needs twenty minutes for the confirmations we require. A ten-minute lock would expire on a large share of honest payments. An hour, on the other hand, roughly doubles the price risk we carry per deposit, which would have to come back into prices somewhere.
Thirty minutes is where the two curves cross. Across the last year, 96% of completed deposits were sent within the window, and most of the rest were exchange withdrawals that the exchange itself held.
What the window covers, precisely
The lock applies to the moment your transaction is seen on the network, not to when it is fully confirmed. If your wallet broadcasts at minute 28 and Bitcoin takes 40 minutes to confirm it, you are inside the window; the page switches to confirming and the locked amount stands. The countdown is the deadline for pressing send, not for the miners.
Paying late
A payment that reaches the network after the window is not lost and is not refused. The address still belongs to your deposit, the provider still sees the funds, and the deposit completes at the market rate of the moment the transaction is first seen. In practice the difference is a few cents in either direction. If the market moved sharply against you, the balance shows a slightly smaller dollar amount than the page originally promised; if it moved in your favour, slightly more. There is no penalty and no support ticket needed.
Underpaying and overpaying
Wallets sometimes deduct the network fee from the amount you typed, which delivers a little less than the locked amount. The deposit then shows underpaid with the exact shortfall, and sending the remainder to the same address completes it; the original lock still applies to the total if the remainder arrives within the window, and the late rule applies otherwise. Overpayments are credited in full at the same rate. We keep neither.
Why not just float the price
A floating price would mean the page shows an amount that changes while you look at it, that your wallet might already have copied, and that would be wrong by the time the network sees it. Every design like that ends with a rule about which price applies anyway, only hidden. A fixed amount with a visible clock is the honest version of the same thing.
The balance does not expire
The lock is on the deposit, not the money. Once credited, dollars in your balance stay dollars: no expiry, no conversion back, no drift with the market. That is the other reason the window can be short: you do not have to pay for a server at the moment you want it. Top up when the fee is low and the price is right, and spend from the balance whenever you like.