Not really. You are looking at manual input or having to hire an actual developer that works both websites and Unreal in order to create a custom API that passes the calls over to the shopify side (over a secure ssl connection if possible).
likely, the hardest or impossible part is assigning the modified product to the client account. Especially with the latest API changes they made.
Btw, why shopify of all things? They make a killer percentage off sales. You would be $way$ better off with a custom implementation of just about anything:
Zencart, big commerce, magento, ucommerce, hell even WordPress with WooCommerce is a better idea (albeit worse for who’s making it work).
Also, Auto retailers have their own merchant providers or loan backers… any of the above software can be made to accept payments with a wide variety of electronic systems which are also PCI compliant - in all likely-hood some of the merchant providers that car dealers use would also have pre-made gateways that can work with those software.