When you see that your customer is busy you should never apologize for interrupting. There is a psychological law that makes a customer react to the attitude expressed by the sales person, in the same manner.
"If you are sorry for interrupting me, why are you interrupting me?"
There is nothing mysterious about this psychological law, except the results that come when you put it into effect. It makes sense. Everyone wants to do the appropriate thing. Everyone wants to "rise to the occasion." We act out our parts in accordance with the stage that we find set before us. There is an unconscious urge for the customer to "live up to" the expectations you, the sales person, has of them.
You have the power to set the stage. If you want your customer to be agreeable, act agreeable. Don’t fall into the trap of responding to their actions and attitudes.
If you decide beforehand that a certain customer is going to be difficult to deal with, chances are you will approach them with a negative attitude, with your fists mentally clenched ready to fight.
When you do this, you literally set the stage for them to act on. He or she rises to the occasion. The customer will act the part that you have set for them to act and you come away convinced that they really are a "tough customer," without ever realizing that your own actions and attitudes helped make them one.
In dealing with your customers, you see your own attitudes reflected back to you in their behavior. When you smile, the person in front of you smiles. When you frown, the person frowns. When you shout, the person shouts back.
How do you want your customer to react to you? Friendly and agreeable of course. So the more friendly and agreeable your approach is, the better response you will get.