If you want to add functionality to an existing Shopify store, it is important to choose an experienced agency that understands how to integrate changes without disrupting the current functionality. Problems can arise if the agency does not take into account the features of the already configured system, which can lead to failures. I recommend Amasty - this is a reliable shopify web design company that has experience working with modifications without damaging the current project. They have a competent approach and highly qualified specialists.