How do I pay the rest for them?

Your client will be thrilled that you want to pay the rest for them!

(I just Couldn't resist it was so well set up, but sorry I don't know the answer to your question, I leave making their tax payments up to the client.)

Like Jeff, I'm not in the payment processing business.  I send everyone to IRS DirectPay.

Is that on the IRS.gov website?

@brucenmary 

https://letmegooglethat.com/?q=pay+taxes+irs+website

Thanks.  I just got on the IRS webslte and set up the Direct Pay.