I tried the app you are talking about and it malfunctioned and customers could not check out during an advertised sale. Tech support helped out 2 days later ( we are mainly a weekend business) but I never did get it working the way we needed. It was last year. I can’t help but wonder about Shopify’s strategy for adding features to the default product that will totally wipeout 3rd party apps like Zapiet? I think the added cost of doing something that feels like it should be included makes me constantly search for other platforms that include some basics for a more reasonable price.