[Django]-Django ValueError: Field 'id' expected a number but got 'S'

When querying a ForeignKey field, you ‘normally’ pass an instance of the model. For example:

# Example models
class AnotherModel(models.Model):
    name = models.CharField(...)


class MyModel(models.Model):
    another_model = models.ForeignKey(AnotherModel,...)


# Get the instance
another_model_instance = AnotherModel.objects.get(id=1)
# Use the instance in the query
my_model = MyModel.objects.get(another_model=another_model_instance)

You can however, use __ (double underscore) to ‘hop’ across and query a specific field. For example:

my_model = MyModel.objects.get(another_model__name='Some name')

With the above example, we are querying using the name field on the AnotherModel model. We can use this to fix the query in your view.

# Taken from your payment_complete view
for item in order_items:
    if item.item.is_footwear:
        st = StockQuantity.objects.get(
            item=item.item,
            color__item_color=item.color,
            footwear_size__footwear_size=str(item.footwear_size)
        )
    else:
        st = StockQuantity.objects.get(
            item=item.item,
            color__item_color=item.color,
            cloth_size__cloth_size=str(item.cloth_size)
        )    

Further reading: https://docs.djangoproject.com/en/3.1/topics/db/queries/.