You can send crypto between Coinbase accounts without paying transaction fees by sending to an email address rather than a crypto address.
This can actually result in you being able to send funds between Coinbase accounts for free!
In general Coinbase doesn’t charge fees for sending, but all crypto transactions have fees due to the way crypto transactions work (fees are included with transactions to incentivize miners to add transactions to the blockchain).
Thus in general, when you send to a crypto address, you have to pay fees.
However, Coinbase is a custodial service and they keep the majority of their funds in cold storage, so when you send between your own accounts or send to another user via email, they can credit accounts without having to commit the transaction to the blockchain.
Since sending via email doesn’t require you to use the blockchain and therefore pay a miner fee, you can send crypto to another Coinbase user using email to save on transaction fees.
It may not cost a lot to send crypto, but if you are sending frequently, using this trick can save a pretty Satoshi.