addProduct method
Adds a product to the EACart instance
Implementation
void addProduct({required Product product, double? amount, int? quantity}) {
if (product.payload[EAPropertyKey.PRODUCT_AMOUNT] == null && amount != null)
product.setAmount(amount);
if (product.payload[EAPropertyKey.PRODUCT_QUANTITY] == null &&
quantity != null) product.setQuantity(quantity);
_products.add(product);
payload[EAPropertyKey.PRODUCTS] = _products;
}