I've thought about this a bit more from last night, and if this guy is a businessman, surely he knows exactly what he's doing, and moreover, he knows it's wrong to do it.
I advocate the "oops, I forgot" strategy someone else mentioned. "I'm sorry, but I didn't address your envelope yesterday. I was very busy cutting checks and filing tax information for Owner. Sometimes I wonder why I became a CPA in the first place! Maybe I'll get to it today."
What's he going to do? Complain about you? To whom? Fire you? He has no authority over you.
The interrupting you while you're on the phone has to stop. Hand up, palm out as if you're talking to a small child. "Excuse me, but I'm on the phone." Then physically turn your body away from him and back toward your computer. Even if you don't need to be looking at any documentation while you're on the phone, find something and put it up in front of your face.
It may be this guy got where he is by being both charming and pushy. You too can be charming and assertive. And frankly, it wouldn't be out of line to say, "Mr. Sleazehound, I'm afraid you think I'm your secretary. In fact, I'm a CPA, and my work is very important to Mr. Owner. If you want someone to lick your stamps, you'll have to ask Ms. Secretary or Ms. Receptionist."
Does he normally work for people for free? No? Then neither should he ask you to do it for him.