Hi there,
A prepaid card may look like a credit card, but it is in fact a debit card because purchases are charged to the linked prepaid account rather than paid on credit which is repaid at the end of the month.
For this reason, when you pay for flights booked on the SWISS website, you should enter the prepaid card information in the debit card field instead of the credit card field. You benefit because you do not pay the credit card markup which Swiss charges when you pay by credit card.
Many online merchants do accept prepaid card information in place of credit card information at the checkout. But when your prepaid card number is rejected, try entering it in the debit card field when this is provided.
Best regards from Moneyguru
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Swiss credit card and prepaid card comparison