You expect the price to hold while you pay

Why a quote has to expire

The listing is priced in a currency people understand. The payment happens in a coin that moves against it all day. Somewhere in between, a conversion is done at a moment in time. Whoever holds the position between those two moments carries the risk, and no platform is willing to carry it indefinitely for free. So the quote gets a life measured in minutes.

The plumbing

1. The order is created and the current rate is applied.
2. You get an amount to send and a window in which it counts.
3. You send. Your transaction sits in the mempool until it is included.
4. It is included, then it accumulates confirmations. This is the slow part and it is covered in the confirmation entry.
5. The platform credits what arrived, valued by its own rules.

Steps three and four are outside everyone's control. A low fee at a busy hour can leave a transaction waiting long past the window with nobody having done anything wrong.

How the shortfall appears

• The rate moved between the quote and the arrival, so the value credited is short.
• The fee came out of the amount sent rather than being added on top.
• You typed the coin amount by hand and rounded it down.
• You sent from a service that took its own cut in the middle.

The result looks identical in all four cases. The order shows a shortfall and waits, and now you are in a slow conversation with a queue rather than in a completed order.

What actually prevents it

1. Have coins ready before you create the order. Funding after the quote starts is what puts the wait inside the window.
2. Send with a fee that gets you into the next block or two, not the cheapest possible.
3. Copy the amount rather than retyping it, the same rule as with addresses.
4. Make sure the fee is added to the total rather than taken out of it.
5. If the window lapses, do not send twice. A second payment is a second problem. Open the order and say what happened.

One more thing worth knowing. Overpaying by a small amount is usually the cheaper mistake, because a shortfall costs days and a small overage costs a small overage. That is a judgement, not a rule, and it is the judgement most experienced buyers make.