Examples on how to pay fees with custom assets using the Asset Conversion Pallet and the Asset Conversion Tx Payment Pallet.
The example using Polkadot-JS API can be found explained here.
The example using Txwrapper can be found explained here.
The example using Subxt can be found explained here.
The example using the ATA can be found explained here.
NOTE: All examples were ran against clients v1.6.0