G: Combined Listings & Variant

This app is by far the best one I've used for combining listings in a fluid and integrated way within Shopify, and it out performs all of the paid ones that I have tried my a long way. On top of that, the support is fantastic! They've been working with me to get it to work with and look integrated into my third-party theme, being very communicative and quick to respond. If you download it to try and it down't seem to work with your theme, definitely reach out to support before trying something else, they will be able to help with getting it work and helping to align it with your theme's design.