Since the items you see on Swappable are backed by the blockchain, payment is also made in blockchain-based currencies, or cryptocurrencies. The most popular form of payment is Ether, which is the token that helps power the Ethereum blockchain Swappable is built upon.
Swappable also supports the TrustSwap SWAP token, and what's more, when paying any fees in SWAP, you will enjoy a 50% discount. 80% of the fee goes to the staking rewards pool and the other 20% is burnt. It is currently not possible to use fiat currencies on Swappable.