If you are comfortable trusting Bitrefill to hold some funds on your behalf, then you can always top up your account with some bitcoin when transactions fees are low, which will allow you to purchase cards instantly at a future date. They have also introduced a "Bitrefill Balance Card" denominated in USD which you can buy using bitcoin. This effectively allows you to convert bitcoin to a USD balance when the price is favorable (either by transferring bitcoin at that time or by spending bitcoin in your account), which you can then spend on cards later.