I think it is acceptable to send out within 3/4 working days - its not really fair to expect someone to go out of their way to get to a post office for 1 item, it would be different if it were a shop, or even an auction that ended at a certain time. The seller can't know when someone will buy it so they can't plan when they will be able to post out.
What postage did she say she would use?
First class I tend to allow get things by the 2nd day after they post, and second class normally by the 3rd day, obviously you need to know when she posted to guess at when it might arrive!