Burn, Claim & Mint on behalf in 1 tx

SNXClaimerZap.sol

pragma solidity >=0.4.22 <0.7.0;

interface ISynthetix {
    function burnSynthsToTargetOnBehalf(address burnForAddress) external;
    function issueMaxSynthsOnBehalf(address issueForAddress) external;
    function remainingIssuableSynths(address issuer) external returns (uint256);
}

interface IFeePool {
    function claimOnBehalf(address claimingForAddress) external;
    function isFeesClaimable(address account) external returns (bool);
}

/**
 * @title Burn, Claim & Mint on behalf in 1 tx
 * 1. Delegate this contract address in mintr.synthetix.io 
 * 2. Call burnClaimMintSNX(your SNX account adddress)
 */
contract SNXClaimerZap {
    address synthetixProxy = 0xC011a73ee8576Fb46F5E1c5751cA3B9Fe0af2a6F;
    address feePoolProxy = 0xb440DD674e1243644791a4AdfE3A2AbB0A92d309;
    
    ISynthetix synthetix = ISynthetix(synthetixProxy);
    IFeePool feePool = IFeePool(feePoolProxy);

    function burnClaimMintSNX(address msg.sender) external returns (uint256) {
        
        if (!feePool.isFeesClaimable(delegator)) {
            synthetix.burnSynthsToTargetOnBehalf(delegator);
        }
        
        feePool.claimOnBehalf(delegator);
        
        if (synthetix.remainingIssuableSynths(delegator) > 0) {
            synthetix.issueMaxSynthsOnBehalf(delegator);
        }
        
        emit SNXClaimerZappedForAccount(delegator);
    }
    
    event SNXClaimerZappedForAccount(address delegator);
}

If you removed address delegator and used msg.sender instead, it would prevent third parties from being able to initiate the ZAP on your behalf, which while they'd pay the gas, it could cause you to have claimed when you weren't prepared to...

Also, you should use external over public for gas savings.