The year I offered "Fee Collect", no one used it. I do have a button on my web site for PayPal payments (client does NOT have to have an account, but it's the only way I'll take a credit card).
My engagement letter says clearly "Due Upon Presentation" but of course there are always one or two that delay, and I generally efile the returns anyway. For the most part they are good clients who have forgotten their checkbook that day. The couple who, in the past, haven't, I've "tweaked" their tax letters in January to say that all past-due balances must be paid before new work can begin. That usually brings them into line.
I'd give them each a call -- or, if you can, email (or fax) them an invoice, marked "PAST DUE" (don't waste postage), and see if that shakes any payments loose. Then perhaps go the "removing my name as "paid preparer" letter" route with the ten-day response. Those would have to be mailed, so that's why I'd try the email/fax routine first.
Good luck! It is SO annoying when people don't pay.
Catherine