I believe those who tried to sneek in the 30% are having the orders cancelled.
Those who have cancelled orders/card decline, can you confirm your order contains 30% discount?

Mine was only 15%, went through fine without a problem, using HSBC.
Natwest is just a b1tch sometime, but they did stop a fraudlent transaction for me once, they sent a text saying there was a suspecious transaction, told me to call them to confirm. I said I didn't authorised it, they did a full refund and sent me a new card.