Hi, @dburney !
Julie here from Shopify Support.
Assuming you’ve created a development store through your Shopify Partners account, you can transfer ownership through your Partner Dashboard. Once you’re ready to launch, your client can transfer their domain to the new store.
Have the new products already been added to your client’s existing store? If so, they can export those products and import them into the new store. Or, they can simply add them to the new store as they would regularly add products.
It’s not possible to import orders into another store, so I suggest having your client keep the old store open until they’ve fulfilled all open orders. Once that’s done, they can export their orders to a CSV file for their records.
Aside from exporting their orders, there may be some other data they may want to consider saving if they plan on closing this store down. For example, they might want to export their billing history. If they sell gift cards, then they’ll want to also export their gift cards so they have the information they may need to resolve any outstanding cards. Customer lists can also be exported and imported into the new store.
We actually have a help doc on how to close your store that offers some steps your client should consider taking before closing their store. Our help doc on backups and duplication also outlines what can be exported into CSV files and what information can be duplicated and transferred to the new store.
I hope this helps. If I’ve misunderstood your question or if you have further questions, please let me know!