As oligopolies, MasterCard and Visa, have tremendous regulatory powers, getting to choose who can use their ubiquitous and essential services. Online casinos and companies that sell porn videos and legal psychedelics like salvia divinorum and kratom use shell companies to hide from credit card networks, but they eventually get caught. That's why many merchants of banned services and companies are starting to accept cryptocurrencies, which aren't controlled by any central authority, and eChecks, the facilitators of which have a much more laissez-faire policy than the credit card networks.
Almost all of the kratom sites, including those that haven't been able to sneak themselves into the card networks, accept cryptocurrencies including Bitcoin, Ethereum, Litecoin, XRP, Stellar Lumens, or some other one. Third-party crypto processors like CoinPayments or Coinbase Commerce are typically used for payments processing.
When they accept cards, kratom sites often offer discounts for cryptocurrency payments. For instance, Happy Hippo's checkout page offers a 20% discount:
It's easy to understand why kratom sites would offer such discounts. It's expensive to use overseas aggregators for card payments. By steering a customer to Bitcoin or Ethereum, a kratom vendor saves itself the pain of a 10-15% card processing fee.