From my it sounds like there could be a problem there and I am sorry you encountering issues. I did a little investigating to learn the following.
The payment processing is different or subscription and no subscription orders. Switching to the other may let you card go through (though at a different price). If you indicate your order type that might make it a little easier for the Huel folks to investigate.
This does not sound like your problem but may be relevant for for future readers (I learned this trying to investigate your issue without actually making an order): prepaid cards will only work with non-subscription orders. If you try one with a subscription order you will get the message:
Processor stripe says: Prepaid cards are not allowed.