Depending on the network chosen by users to stake their Matic with Stader, they would need to pay gas fees in ETH or MATIC as applicable. Stader does not charge any additional transaction fees.
Stader doesn't levy any charge to users on Deposits/Staking
Stader will charge a protocol fee of 10% of the rewards added to the contract. We use it for upgrading our infrastructure to develop quality products for our users. Treasury for Polygon MaticX is a multi-sig (0x80A43dd35382C4919991C5Bca7f46Dd24Fde4C67) with 2 threshold signatures on 3 signers completely owned by Stader.