Paid tax preparer question: If I do taxes for a client who is paying me for my services, how do I efile for the client? Is it free?

Paid tax preparer question: If I do taxes for a client who is paying me for my services, how do I efile for the client? Is it free? Maybe just use free fillable forms, the free filing service provided by the IRS. (I assume that paid tax preparers could use it too.)

(I realize that TT is not for use for paid preparers. I just don't know where else to ask on the internet.)