For buy offers, rates are usually floating, though some partners do offer fixed rates when a customer pays immediately or can prove that they have paid (usually in the case of card payments).
For exchanges, float vs. fixed is clearly marked. Floating rate exposes the customer to the exchange rate fluctuation, which may end up better or worse for the customer. Especially during the time of high blockchain fees or congested mempool, this can lead to large differences between what was promised and what is received.
Fixed rate is more expensive, but the exchange rate fluctuation risk is borne by the partner.