Occasionally, the amount that appears on a customer's card may exceed the purchase price of the subscription. Usually this is a very small amount, for example 1%. Some banks impose additional fees for certain cards or transaction types, and some US banks add a surcharge when purchasing from a non-US entity, even if the currency is USD. The additional charge does not originate from either HIBP or Stripe.
If you find an unexplained surcharge on your transaction, check with your bank as to the circumstances that have caused it to be added.