The market rate at credit is the exchange rate applied when a confirmed crypto deposit is converted into the USD balance; from that moment the balance is fixed in dollars and no longer follows the coin's price.
A gateway that prices in dollars needs a moment to fix the value of a deposit. The fair moment is confirmation: the rate displayed at that instant converts the coin amount into dollars, once. After that, the coin can rise or fall without changing what the balance buys, which is what a business wants from a budget.
The alternative, holding balances in the coin, makes every campaign cost unpredictable.
At SMSMeteor: deposits are converted at the displayed market rate at confirmation and held in USD; the rate card is in USD too, so a balance of 250 buys exactly what the calculator shows.