How do I parse out Smartsheet row data from an API call for use as line items?

@Troy Tessalone having said that, using a combination of the code generator in Zapier, some refinement from ChatGPT, and a final bit of fettling from my side, I have a working solution which looks like this:

Code-wise, step 2 looks like this:

# Importing the requests library to make HTTP requests
import requests

# Defining the URL for the API call. Replace [Your Sheet ID] with the actual Sheet ID.
url = "https://api.smartsheet.com/2.0/sheets/[Your Sheet ID]"

# Defining the headers for the API call. Replace [Your Access Token] with the actual Access Token.
headers = {
    "Authorization": "Bearer [Your Access Token]",
    "Content-Type": "application/json",
}

# Making a GET request to the Smartsheet API to retrieve the data from the specified sheet
response = requests.request("GET", url, headers=headers)

# Converting the response data to JSON format
data = response.json()

# Initializing an empty dictionary to store the grouped data
grouped_data = {}

# Looping through each row in the sheet
for row in data["rows"]:
     # Checking if the row contains the specified value. If not, the row is skipped. Note the value 3 refers to the fourth column from the left in the sheet, which is where the Invoice Number is found
    if row["cells"][3]["value"] != input_data["invoiceNumber"]:
        continue
    # Looping through each cell in the row
    for cell in row["cells"]:
        # Checking if the 'value' key is present in the cell dictionary
        if "value" in cell:
            # Checking if the column ID of the cell is already a key in the grouped_data dictionary
            if cell["columnId"] in grouped_data:
                # If the column ID is already a key, append the cell value to the list of values for that key
                grouped_data[cell["columnId"]].append(cell["value"])
            else:
                # If the column ID is not a key, create a new key-value pair in the grouped_data dictionary
                # The key is the column ID and the value is a list containing the cell value
                grouped_data[cell["columnId"]] = [cell["value"]]

# Storing the grouped data in the output variable as a dictionary
output = {"grouped_data": grouped_data}

Thanks for the pointers!