def add_item(self, item_id, description, amount, quantity, shipping_cost=None, weight=None):
item = Item(
item_id=item_id,
description=description,
amount=amount,
quantity=quantity,
shipping_cost=shipping_cost,
weight=weight,
)
item.validate()
if not self.payment_request.checkout.items:
self.payment_request.checkout.items = []
self.payment_request.checkout.items.append(item)