I always send the paper vouchers with a note that I suggest paying online to have confirmation (or this year, to make sure it gets processed any time soon!) and the links. Unless I'm sure they'll pay online, I send vouchers WITH addressed envelopes.
I will have to admit, this is a different approach and it would avoid the late filing fees and penalties on the federal and state quarterly reports.
However the taxpayer is an employee, not an independent contractor.