Are you an AICPA member or CPA? Regardless, it's a good starting point and most state licensing boards follow their guidelines. Generally speaking, if it is an original document you must return it to the client, even if they ask you to mail them. You may request they pay postage, but after 45 days you still have to return them even if they don't pay. You cannot hold original documents ransom for payment unless allowed by state law.
See this link, start reading on page 12. Then look at your state requirements.
https://us.aicpa.org/interestareas/professionalethics/resources/tools/downloadabledocuments/ethics-general-faqs.pdfI found Q&A #6 pretty interesting, but I would not rely on it!
Also read Section 10.28 of IRS Circular 230.
I have discussed this with lawyers and the general consensus is it is not worth the potential headache of creating acts discreditable with AICPA or violating state or Federal requirements. Because I do not trust mail services or common carriers for such documents, every single tax client signs an indemnification agreement my lawyer drafted in case they request (or I am required to) send physical copies of anything.