After installation registration, when I receive response from UPS, it marshalls categories into this JSON snippet:

"categories": [
    {
        "id": null,
        "name": "category_name"
    },
    {
        "id": null,
        "name": "another_name"
    },
]

It is questionable, what purpose that null field fulfils if it is always null.

There is not automatic generation of ID on that Category model via uuid on id property so even DB layer saves it and some ID is assigned to it, it is never set back in model.

Try to marshall Category into JSON in such way only name of category is used and all categories are marshalled into array of names and not into useless hash map (difficult to parse).