ERC-20 tokens are smart contracts executed on the Ethereum network. Therefore, transactions of ERC-20 tokens such as PAX or USDC require that the transaction fees (or gas) are settled in ETH. 

To initiate a new transaction of an ERC-20 token, the source wallet you are sending from must have a minimum balance of 0.0005 Ethereum (ETH) to instantiate the operation.  

Currently, Balance Custody retrieves the default (or "standard") gas price for transactions and applies that value dynamically to each operation on submission. You can see the up to date average transaction fees in GWEI by visiting https://ethgasstation.info/