That is because the products are commodities and are presumably identical. The question here is whether or not the
signs we make and sell are a commodity or a custom made product. My opinion is that they can be either and that the business model of the OP is his free choice. It may be unnecessarily aggressive towards his competition and the price of reducing or eliminating competition may be more than he realizes ... but still his choice to make.
Those who choose a business model that emphasizes their skill, their quality or their ability to produce better designs so as to provide better client results should not be concerned. Those who choose to market signage in competition with the OP who are unable or unwilling to provide a superior product will have problems and may need to reconsider their business model.