Rounding rule

...
"shopping_cart":{
  "items":[
    {
      "name":"Product 1",
      "description":"",
      "unit_price":13.761467889,
      "quantity":2,
      "merchant_item_id":"00001",
      "tax_table_selector":"BTW9",
      "weight":{
        "unit":"KG",
        "value":12
      }
    },
    {
      "name":"Product 2",
      "description":"",
      "unit_price":8.2644628099,
      "quantity":4,
      "merchant_item_id":"00002",
      "tax_table_selector":"BTW21",
      "weight":{
        "unit":"KG",
        "value":1
      }
    },
    {
      "name":"Shipping",
      "description":"",
      "unit_price":4.5412844037,
      "quantity":1,
      "merchant_item_id":"msp-shipping",
      "tax_table_selector":"BTW9",
      "weight":{
        "unit":"KG",
        "value":0
      }
    }
  ]
},
"checkout_options":{
  "tax_tables":{
    "default":{
      "shipping_taxed":true,
      "rate":0.21
    },
    "alternate":[
      {
        "name":"BTW21",
        "standalone":true,
        "rules":[
          {
            "rate":0.21
          }
        ]
      },
      {
        "name":"BTW9",
        "standalone":true,
        "rules":[
          {
            "rate":0.09
          }
        ]
      },
      {
        "name":"none",
        "standalone":false,
        "rules":[
          {
            "rate":0
          }
        ]
      }
    ]
  }
}
...

# Step 1: Calculate the total cost of all items in the cart (excluding VAT). Multiply the unit_price of each item by the quantity of the item, then add the results together, rounding to 2 decimal places: items_total.
items_total = 0
for item in order.shopping_cart.items:
  items_total += item.unit_price * item.quantity
items_total = round(items_total, 2)

# Step 2: Calculate the total VAT for all items in the cart (if applicable). Multiply the total cost of each item by the VAT rate. Add the results together, and round to 2 decimal places: vat_total.
vat_total = 0
for item in order.shopping_cart.items:
  for tax_class in order.checkout_options.tax_tables.alternate:
    if tax_class.name == item.tax_table_selector:
      vat_total += item.unit_price * item.quantity * tax_class.rules[0].rate
      break
  else:
    vat_total += item.unit_price * item.quantity * order.checkout_options.tax_tables.default.rate
vat_total = round(vat_total, 2)

# Step 3: Add the items total to the VAT total: cart_total.
cart_total = items_total + vat_total

// Step 1: Calculate the total cost of all items in the cart (excluding VAT). Multiply the unit_price of each item by the quantity of the item, then add the results together, rounding to 2 decimal places: itemsTotal.
let itemsTotal = 0;
for (const item of order.shopping_cart.items) {
  itemsTotal += item.unit_price * item.quantity;
}
itemsTotal = Math.round(itemsTotal * 100) / 100;

// Step 2: Calculate the total VAT for all items in the cart (if applicable). Multiply the total cost of each item by the VAT rate. Add the results together, and round to 2 decimal places: vatTotal.
let vatTotal = 0;
for (const item of order.shopping_cart.items) {
  for (const tax_class of order.checkout_options.tax_tables.alternate) {
    if (tax_class.name == item.tax_table_selector) {
      vatTotal += item.unit_price * item.quantity * tax_class.rules[0].rate;
      break;
    }
  }
  if (vatTotal == 0) {
    vatTotal += item.unit_price * item.quantity * order.checkout_options.tax_tables.default.rate;
  }
}
vatTotal = Math.round(vatTotal * 100) / 100;

// Step 3: Add the items total to the VAT total: cartTotal.
cartTotal = itemsTotal + vatTotal;

# Step 1: Calculate the total cost of all items in the cart (excluding VAT). Multiply the unit_price of each item by the quantity of the item, then add the results together, rounding to 2 decimal places: items_total.
items_total = 0
order.shopping_cart.items.each do |item|
  items_total += item.unit_price * item.quantity
end
items_total = items_total.round(2)

# Step 2: Calculate the total VAT for all items in the cart (if applicable). Multiply the total cost of each item by the VAT rate. Add the results together, and round to 2 decimal places: vat_total.
vat_total = 0
order.shopping_cart.items.each do |item|
  tax_class = order.checkout_options.tax_tables.alternate.find { |tax_class| tax_class.name == item.tax_table_selector }
  if tax_class
    vat_total += item.unit_price * item.quantity * tax_class.rules[0].rate
  else
    vat_total += item.unit_price * item.quantity * order.checkout_options.tax_tables.default.rate
  end
end
vat_total = vat_total.round(2)

# Step 3: Add the items total to the VAT total: cart_total.
cart_total = items_total + vat_total

// Step 1: Calculate the total cost of all items in the cart (excluding VAT). Multiply the unit_price of each item by the quantity of the item, then add the results together, rounding to 2 decimal places: items_total.
var items_total = 0;
foreach (var item in order.shopping_cart.items) {
  items_total += item.unit_price * item.quantity;
}
items_total = Math.Round(items_total, 2);

// Step 2: Calculate the total VAT for all items in the cart (if applicable). Multiply the total cost of each item by the VAT rate. Add the results together, and round to 2 decimal places: vat_total.
var vat_total = 0;
foreach (var item in order.shopping_cart.items) {
var tax_class = order.checkout_options.tax_tables.alternate.FirstOrDefault(tax_class => tax_class.name == item.tax_table_selector);
if (tax_class != null) {
  vat_total += item.unit_price * item.quantity * tax_class.rules[0].rate;
} else {
  vat_total += item.unit_price * item.quantity * order.checkout_options.tax_tables.default.rate;
}
}
vat_total = Math.Round(vat_total, 2);

// Step 3: Add the items total to the VAT total: cart_total.
cart_total = items_total + vat_total;

// Step 1: Calculate the total cost of all items in the cart (excluding VAT). Multiply the unit_price of each item by the quantity of the item, then add the results together, rounding to 2 decimal places: items_total.
var items_total = 0
for _, item := range order.shopping_cart.items {
  items_total += item.unit_price * item.quantity
}
items_total = math.Round(items_total, 2)

// Step 2: Calculate the total VAT for all items in the cart (if applicable). Multiply the total cost of each item by the VAT rate. Add the results together, and round to 2 decimal places: vat_total.
var vat_total = 0
for _, item := range order.shopping_cart.items {
  tax_class := order.checkout_options.tax_tables.alternate.Find(tax_class => tax_class.name == item.tax_table_selector)
  if tax_class != nil {
    vat_total += item.unit_price * item.quantity * tax_class.rules[0].rate
  } else {
    vat_total += item.unit_price * item.quantity * order.checkout_options.tax_tables.default.rate
  }
}
vat_total = math.Round(vat_total, 2)

// Step 3: Add the items total to the VAT total: cart_total.
cart_total = items_total + vat_total