I do know that you have to REP to file an extension.

I'm not 100% sure, but I don't think you can only REP Federal if states exist. I agree deleting them would be bad, but I don't know of another workaround.

If your firm model / client demographic allows for this, consider charging for extensions to at least cover your REP fees, even if you credit the extension billing against their final bill.

Otherwise, just call the REP fees a "cost of doing business," and think of the payroll & time savings from 1) not generating & mailing paper extensions, and dealing with the resulting IRS letters.