Since Shopify doesn't offer a feature for us to do update "Checkout Line Items Properties" directly on the Admin Panel I'm trying to do it through the Shopify API. So I started testing a simple Checkout creation:
// Create without line_items (which is an item marked as required in the documentation) it work:
I tested several different paths and options (I could describe it but it would be too long) but nothing worked. Even the simplest updates do not work for checkout (but they are other cases and deserve to be reviewed separately).
As also noted in other requests, when using the Shopify gem (or another resource) the "variant_id" is converted to a number and the above error is returned. But when we perform the search manually and enter the "variant_id" as a string when updating a Checkout, the request is successful.
There may have been some misunderstanding on my part about the documentation, but I leave these observations eventually to anyone who experiences the same difficulty.
I took a look into this but I was unable to reproduce the behaviour you described in your post. Are you still experiencing this? If you are, could you please provide me with replication steps so I could investigate further.