You can put a bandana between the bike frame and the post you are locking it to.
I lock my bike to a wrought iron park bench sometimes on Main St where there are no parking sign poles. There is almost always SOMETHING you can lock to if you think outside the box. Do ask the store to consider a bike rack- tell them more and more people are biking to save gas and you will shop there more often if you can lock up your bike outside safely.