Can a 3 day notice to tenant be served to someone at work?

My husband and I rent a home to a couple and everything was going smoothly until this month. My husband actually worked with the lady and they were friends.(first mistake....I told him!) He offered our home to her and her fiance' for a very good rate. It was a verbal month to month agreement. They have rented the home over a year without missing any payments, but this month when my husband text the lady about rent she blew him off. After a couple of weeks she finally texted him back saying that her and her fiancee' is buying a new home and they don't have the money to pay us because they are closing on the 27th of this month. My husband didn't want to pursue anything, he just wanted them out, but I knew there's no way they would be closing on a home in 2 weeks. I was right, and they are still in the house, so we sent a certified letter with a 3 day notice to tenant pursuant to chapter 83.56(3). She didn't answer the door and there was a notice left in her mailbox from the mail carrier telling her she had a certified letter to pick up. She refuses to pick the letter up at the post office, has ignored any text messages, and removed my husband from facebook. She's obviously trying to avoid us at all cost. We plan on leaving the 3 day notice on the house tomorrow and my husband also wants to drop off a copy to her place of employment. He works in the same field as her, so he knows a lot of the staff. He wants to make sure someone witnesses her recieving the letter. I just wanted to make sure this was legal. I can't seem to find anything regarding this, only that the 3 day notice has to be delivered to the person or left on the house. I know if I mail it she will say she never received it, and she can do the same if we just leave it on the house. She'll just say the wind must have blown it off. This is a sure way of having witnesses that she received the letter from us. I just don't want to go do anything that is illegal or makes us look bad if we end up in court.