Tough Customer W1 10/11
to be clear you went to the shop thats at the top of that tall log with lots of bouncepads in the inside? Going in should have triggered it if you did. If that isnt the one you went in I recomend looking up the dulfy guide for where all the shops are.
http://dulfy.net/2013/09/10/gw2-tough-customer-world-1-achievement-guide/
This is shop #6 in the achievment list as of 2016. The dulfy guide isnt updated and has shop #6 as the waterfall shop. Im still looking for this one myself.