Hi. I have reread your job post and I understand rough flow of your service.
So I think as follows:
1. For prototyping your service, It's good to use Shopify platform. But depending on my experience of Shopify, it will need several specific apps ( you have to pay for each app monthly ), and its hard to implement functions that is specific to your service. ( for example, 2, 3, 4 functions in your main features ).
2. Anyway, your service is much like eCommerce, so I think it's also good to implement it from scratch using well-known PHP frameworks or Woocommerce on Wordpress.
Then, its easier to implement specific functions than above ones.
So I want to know more details on your service to confirm needed features.
Please discuss that problems.
Regards.