CashDash always negotiates the best possible market rates, and will lock them down for you whenever available.
Lockdown is time limited, which is why you’ll see a running timer under each new quote when you buy cash or top up your CashDash account.

No need to worry, though.
You’ll still be able to top up funds and buy cash when the timer runs out, though the rate may have changed by then – sometimes even for the better!