I've mailed to various entities over the years via certified mail, and sometimes it took a while for the signed postcard to come back. I didn't worry about it because I had the postmark stamped certified receipt from when I mailed it. And I never had any problems with late filing fees or anything else. You have proof it was mailed, so I wouldn't worry about it. I've also used USPS Priority Mail over the years to mail the clients and it is excellent. Although I am not saying this about your clients, sometimes you can't believe everything that a client says. During this tax season a client texted me on a Friday and claimed she did not get her tax returns. I checked on USPS online and it was delivered in her mailbox on Tuesday. I sent this information back to her via text. She replied: "I just found it in the mailbox". Yeah right it sat there from Tuesday to Friday and she never checked her mail. She just wanted me to call her back so she could pump some more free information out of me.