As your parents are US Citizens I dont think there will be a problem but they might face some difficulties while going out as the visas are not stamped in their Passports. They might have entered their names in their system and they will not stamp the visa on the passport unless you pay the fee. The card did not work but they dont want to hold US Citizens at the Airport to create an issue. It will be better to go there and pay the fee and get the visa stamped on their Passports.