The ten-minute quote hold, and why it exists
A number that can move while you look at it is not a quote.
When Topsy gives you a fare, that fare is held for ten minutes. If the window lapses, the trip is re-priced in front of you before you can book.
Both halves of that are deliberate and both took argument to settle.
Why hold it at all
Because deciding takes time, and none of the reasons are unreasonable. You are checking whether a colleague is coming. You are comparing an Economy with an XL. You are finding your shoes.
A quote that changes silently while you do those things is not a quote. It is an estimate that reserves the right to be different, and a customer who discovers that once will not trust the next one.
Why ten minutes rather than an hour
Because a fare reflects conditions, and conditions move. Traffic changes, demand changes. Holding a price for an hour means either quoting conservatively for everyone to cover the risk, or honouring stale prices that no longer reflect the road.
Ten minutes is long enough to decide and short enough to still describe the journey you are about to take.
What happens when it lapses
The quote expires and is recalculated. Crucially, you are shown the new number and asked again rather than being charged the new one against a decision you made on the old one.
That is the part that matters. Expiry is not a penalty; it is a refusal to book you onto a price you did not agree to.
Where you see it
The quote panel carries the time the fare is held until. If you have been on the page a while, that is the thing to glance at before pressing book.
The general rule
Any number a customer is asked to decide on should be stable while they decide, and any change should be surfaced rather than applied. Almost every pricing complaint in this industry is a violation of one of those two.