Actually, Tags would be more useful to us. Metafield data is not accessible until after the order has been placed. Plus, it just seems to dump all the metafields together in a string.
While I appreciate the effort, it would be nice to be able to get some data to Shipstation at the time of order placement as opposed to having to save the order, then edit it to add the metafields, then save again. If we have to use metafields, one idea would be to have a metafield definable on the Shopify end, that we would purposely dedicate to Shipstation. As in a metafield naming convention like, "ss_custom_field_1" and then pull them from there. Even better would be to be able to pull from the product metafields in line items of the order.
THAT would be a good use of mapping metafields to the Custom fields. If associated with the product, all kinds of useful info could be placed there. Warehouse shelf/bin information comes to mind.
Thank you for inquiring about the status of this feature request. While it is not currently on the roadmap, the team has informed me that they are beginning the initial research to determine what it would take to get Metafields from Shopify mapped in ShipStation.
We have recently implemented a new feature that should be helpful for you. This feature is currently enabled by support so you would have to reach out to them or reply to this comment with your ShipStation username. We have a setting that when enabled will import in 'Product Metafields' into ShipStation. This will show up as item 'options' will be displayed on item level basis as you were suggesting in your post above.
We do have a 500 character limit therefore ensure the metafield that you want to import into ShipStation is on top of the list inside of Shopify.