Edge Cases and Tenfold Scale

Odeko turns each day's coffee shop orders into purchase orders sent to suppliers. Here is the function that rolls up every café's line items into per-supplier case counts.

def build_purchase_orders(daily_orders, supplier_catalog):
    purchase_orders = {}
    for order in daily_orders:
        for line in order["items"]:
            sku = line["sku"]
            quantity = line["quantity"]
            supplier = None
            for catalog_entry in supplier_catalog:
                if catalog_entry["sku"] == sku:
                    supplier = catalog_entry["supplier"]
                    break
            if supplier not in purchase_orders:
                purchase_orders[supplier] = {}
            if sku not in purchase_orders[supplier]:
                purchase_orders[supplier][sku] = 0
            purchase_orders[supplier][sku] += quantity
    for supplier, skus in purchase_orders.items():
        for sku, total in skus.items():
            case_size = 12
            skus[sku] = total // case_size
    return purchase_orders

How does this solution handle edge cases, and how would you scale this implementation if the number of cafés and daily orders increased tenfold? Walk through what you would check for and what you would change.