Is it permissible to buy medicine for a sick mother, whose recovery is not expected by doctors, using a "Visa card" while being careful to avoid usury, or must it be bought at a double price through another agent, knowing that the currency exchange difference is significant?

The covered Visa card is permissible if used within the limits of its balance without a loan or late fees. If a covered card is not available, there is no harm in using an uncovered card, provided one promptly repays what has been spent from its balance to avoid usurious penalties, and ceases using it once the need for it is over.