How to set Invoice Tax Code in SuiteScript 2.1

I'm having some issues with setting the Tax Code on a Line Item related to an Invoice, this is the JSON for 1 of the Invoice Items I am passing to a RESTlet:

    {
        "item": {
            "items": [
                {
                    "taxCode": "8",
                    "quantity": 1,
                    "item": "7",
                    "description": "description",
                    "amount": 2280.00
                }
            ]
        }
    }

The SuiteScript parses the JSON and sets the fields where necessary, I am able to set every other type of field, apart from the tax code, I have tried passing the Id as String and an Integer, as well as passing it as an object, like the below 2 examples, but I keep on getting errors no matter what I do.

"taxcode": {
    "items": [
        { "id" : 8 }
    ]
}
"taxcode": {
    { "id" : 8 }
}

This is my RESTlet SuiteScript that iterates over the items and sets the relevant field values, as mentioned above; this is working for all other fields, it's just the tax code I am having issues with

for (let i of req[cust][key].items) {
    // Select the sublist line
    invoice.selectNewLine({
        sublistId: key.toLowerCase()
    });

    // Iterate over the fields
    for (let j in i) {
        try {
            // If the current field is an object
            if (i[j] instanceof Object) {
                // Get the subrecord
                var child = invoice.getCurrentSublistSubrecord({
                    sublistId: key.toLowerCase(),
                    fieldId: j.toLowerCase()
                });

                // Set it's fields field
                for (let k in i[j]) {
                    // Set the field and value
                    child.setValue({
                        fieldId: k.toLowerCase(),
                        value: i[j][k]
                    });
                }

                // Commit the line
                invoice.setValue({
                    fieldId: j.toLowerCase(),
                    value: child
                });
            } else {
                log.debug(
                    "setCurrentSublistValue",
                    JSON.stringify({
                        sublistId: key.toLowerCase(),
                        fieldId: j.toLowerCase(),
                        value: i[j]
                    })
                );
                // Set the fields
                invoice.setCurrentSublistValue({
                    sublistId: key.toLowerCase(),
                    fieldId: j.toLowerCase(),
                    value: i[j]
                });
            }
        } catch (e) {
            log.debug(127, j);
            log.debug(128, e);
        }
    }

    // Commit the line
    invoice.commitLine({
        sublistId: key.toLowerCase()
    });
}

Any ideas?

WORST API EVER!

You have to set the TaxCode last... or something else that is being set wipes it out -_-

Doesn't work:

{
    "item": {
        "items": [
            {
                "taxCode": "8",
                "quantity": 1,
                "item": "7",
                "description": "description",
                "amount": 2280.00
            }
        ]
    }
}

Works perfectly fine:

{
    "item": {
        "items": [
            {
                "quantity": 1,
                "item": "7",
                "description": "description",
                "amount": 2280.00,
                "taxCode": "8"
            }
        ]
    }
}