Just in time

At first sight it looks as if the lady has rushed up just in time to hand a letter to the postman who is emptying the letterbox.

However, when the postman drew up in his van, he found a note taped to the outside of the letterbox (over the lock, so he couldn't fail to notice it).  At that point, the lady rushed out of the shop and spoke to him. As he emptied the box, he handed one of the letters to her; she rushed into the shop, and then rushed out again and gave him the letter back.  What was that all about?

I'm guessing maybe she forgot to put a stamp on it before posting, and realised too late?  A good job the postman was cooperative.