First, I would recommend DON'T. Emailing is not secure.
Second, does PS Basic have access to Intuit Link? If so, that would be my top recommendation.
If not, your next best option is to email it encrypted, that they'll need a password (which you can provide by phone or text, not in the email obviously) to give them.
If e-signature is working, you might send it that way. If I understand the system (which I'm not sure I do yet), you can just send something to them without signature tags, and they can read it that way?
But re-read paragraph 1. I know with this COVID thing it's tempting to avoid the personal touch, but let's not risk security in the name of health. From all reports I've heard, you can recover from Coronavirus a lot faster than identity fraud!