I'm afraind it is not enough for you to either see the Great wall and also do shopping. 7 Hours seems not enough for only the Great wall, since you'll spend time on the way. 7 hours you can just do some shopping and tour around Tiananmen Square and other attractions. For the stores of North Face, there are many many in Beijing, I suggest you go to Joy City near Xidan, it is convenient for you to access to other attractions after shopping. Otherwise, if you will take flight leaving from Beijing, I think you could do shopping in the airport.