In the end of this section, the key and value of this dictionary should be swapped:

field_name_mapping = {
 'line_items': 'order_items'
}
# should be:
field_name_mapping = {
 'order_items': 'line_items'
}

The language for overriding related_name in a url is a bit confusing and it took me a bit to realize this map was the problem, and wanted to save others the trouble of messing around with this configuration.