You can certainly manually enter data from QB to Shopify or you can use a connector as you mentioned above like Lettuce. We do offer a similar product but provides a full multichannel integration solution to help you sync Shopify with QB and then helps keep inventory, orders and customer data in sync with each system. It will even help sync with your POS system or other. I noticed some people were mentioning entering the data into QB, when you could use an add-on or integration solution to save you time and energy with those processes.
I can answer any questions, we also have information on our Shopify connector page: https://www.nchannel.com/connectors/shopify/
Thanks,
Mollie
nChannel