Depending on my relationship with that LBS, I’d either be speaking reasonably to them or making it clear I was going to start a Small Claims Actions. Supply of Goods and Services, Sale of Goods, Duty of Care etc etc.
The fact that several rocket scientists contributing to this thread have pointed out that it is actually easy to do it yourself (in true, friendly STW fashion) is no defence for the shop, who should know better than to either fek it up or not double check with you if they had doubts. Long live the culture of litigation I say, if a reasonable conversation / comprimise doesnt get you anywhere.
Good luck
Stu (Lawyers4U)
ps – I would have used a permanent marker too!