I note that there is a similar query here.

I'm trying to add a "Contribution" product to the cart. This product is a bit like a donation, and doesn't really exist. I created it like this:

internal class ContributionProduct : IProductSnapshot
{
    public const string ContributionName = "Contribution";
    public const string ContributionSku = "Contrib";

    public ContributionProduct(Guid storeId, Guid? taxClassId, ProductPrice price)
    {
        this.StoreId = storeId;
        this.Prices = new[] { price };
        this.TaxClassId = taxClassId;

        this.Properties = new Dictionary<string, string>() { { MyConstants.Store.ContributionProductIdentifierPropertyName, bool.TrueString } };
    }

    public Guid StoreId { get; }
    public IEnumerable<ProductPrice> Prices { get; }
    public IDictionary<string, string> Properties { get; }
    public bool IsGiftCard => false;
    public string ProductReference => nameof(ContributionProduct);
    public string Sku => ContributionSku;
    public string Name => ContributionName;
    public Guid? TaxClassId { get; }

    public static ContributionProduct Create(OrderReadOnly order, decimal contributionValue)
    {
        return new ContributionProduct(order.StoreId, order.TaxClassId, new ProductPrice(contributionValue, order.CurrencyId));
    }
}

and I add it like this

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult AddContribution(AddContributionDto postModel)
    {
        try
        {
            var store = this.CurrentPage.GetStore();

            using (var uow = this._uowProvider.Create())
            {
                var order = this._sessionManager.GetOrCreateCurrentOrder(store.Id)
                    .AsWritable(uow);

                var contributionProduct = ContributionProduct.Create(order, postModel.ContributionValue);

                var productPrices = contributionProduct.Prices;
                var currencyPrice = productPrices?.FirstOrDefault(x => x.CurrencyId == order.CurrencyId);
                if (currencyPrice == null)
                {
                    Debug.WriteLine($"Can't add a Product to an Order where there is no price for the Order currency.");
                }

                order.AddProduct(contributionProduct, 1);

                this._orderService.SaveOrder(order);

                uow.Complete();
            }
        }
        catch (ValidationException)
        {
            this.ModelState.AddModelError("productReference", "Failed to add contribution to cart");

            return this.CurrentUmbracoPage();
        }

        return this.RedirectToCurrentUmbracoPage();
    }

However I'm getting an exception stating "Can't add a Product to an Order where there is no price for the Order currency." at AddProduct(...) and my Debug.WriteLine(...) is never hit.

I'm a bit lost. Any points appreciated.