So you own a Catholic store and you really wish that more people would shop there. One question you might want to ask is what are you involved in in your diocese that would give more people contact with your store?
If your contribution to the Church is solely showing up on Sunday for Mass and running your store, very few people will know you are there. You have to network with local Catholic organizations and sometimes that means getting involved in them.
For example, your parish may have an apologetics club. Get involved. You will probably learn something about the Faith and at the same time have the opportunity to let people know about your store and also that you can supply the club with material at a discount since they will be buying multiple copies of books.
You could also get involved with a local Serra club (not Sierra) to help support and encourage priestly vocations. This is a great way to get to know your future deacons and priests and develope relationships with them that will benefit both you and them in the future - they know they have a trustworthy source for Catholic products and you are that source.
I also suggest that you advertise on at least a few parish bulletins. This helps out the parish by helping them cover the costs of the bulletin and establishes some goodwill with the pastor. I realize that the return on bulletin ads is very small but it is the goodwill and potential ways you can work with the parish that are important.