Shopify already gets people closer to big sites than was ever possible before, especially with the judicious use of add-ons from their store. It's very likely that the feature you want can be found there. It is possible to write custom code for Shopify and sometimes that's necessary, but the only time I ever did it successfully was for companies that were already $1M+ businesses. They could afford to invest in customization because they had the sales to back it up.