Returns with payment included go to a mail drop address, which varies depending on the state in the taxpayer's address.  PTO should have that address printed on the client letter automatically unless you have entered an override for the service center.

https://www.irs.gov/filing/where-to-file-addresses-for-taxpayers-and-tax-professionals-filing-form-1...

Given all the news about the huge backlog the IRS has with paper return, any particular reason why your client would want to file it on paper?  Similarly, making an online payment may be preferable to sending a paper check.

https://www.irs.gov/payments