Hi Warren

Ok, I'm trying to piece this all together as it's not really one succinct question right now, but I'm assuming you have a product that uses the variants prop editor and for each variant you can provider a price and a pricePerSqMetre for. Then, when you add a variant to your order, you want to be able to provide a sqMetre property stored against the order line. Then, when calculating the price for the order line, if it has a sqMetre property defined, then use that and multiply by the variants pricePerSqMetre to define it's price, otherwise, if a sqMetre property isn't defined, then just use the default price.

So based on this assumption, and assuming your properties are named as per the above, you'd probably need to do the following in your add to cart

// DTO to hold posted values so you should be posting these values back
// from your HTML
public class AddToCartDto
{
    public string ProductReference { get; set; }
    public string ProductVariantReference { get; set; }
    public string SqMetre { get; set; }
}

[HttpPost]
public ActionResult AddToCart(AddToCartDto postModel)
{
    try
    {
        using (var uow = _uowProvider.Create())
        {
            var store = CurrentPage.GetStore();
            var order = _sessionManager.GetOrCreateCurrentOrder(store.Id)
                .AsWritable(uow)
                .AddProduct(postModel.ProductReference, postModel.ProductVariantReference, 1, new Dictionary<string, string> {
                    { "sqMetre": postModel.SqMetre }
                });

            _orderService.SaveOrder(order);

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

        return CurrentUmbracoPage();
    }

    return RedirectToCurrentUmbracoPage();
}

You'll then need to implement a custom IOrderLineCalculator inheriting from the default order line calculator

public class SqMetreOrderLineCalculator : OrderLineCalculator
{
    private readonly IPublishedContentQuery _publishedContentQuery;

    public OrderLineCalculator(ITaxService taxService,
            IStoreService storeService,
            IProductPriceFreezerService productPriceFreezerService,
            IPublishedContentQuery publishedContentQuery)
        : base(taxService, storeService, productPriceFreezerService)
    { 
        _publishedContentQuery = publishedContentQuery;
    }

    public override Price CalculateOrderLineUnitPrice(OrderReadOnly order, OrderLineReadOnly orderLine, Guid currencyId, TaxRate taxRate)
    {
        // See if the order line has a sqMetre property
        var sqMetre = int.Parse("0"+ orderLine.Properties["sqMetre"]);
        if (sqMetre != 0) {

            // Get the product + variant ID's from the order line
            var productKey = Guid.Parse(orderLine.ProductReference);
            var variantKey = Guid.Parse(orderLine.ProductVariantReference);

            // Get the product + variant Umbraco nodes, typed to their Models Builder types
            var productNode = _publishedContentQuery.Content(productKey) as ProductPage; // Assumes you have a models builder model called ProductPage
            var variantNode = productNode.Variants[variantKey].Content as ProductVariant;  // Assumes you have a models builder model called ProductVariant

            // Fetch a per sq metre price for the given currency
            var price = variantNode.PricePerSqMetre.GetPriceFor(currencyId);
            if (price != null && price.Value > 0) 
            {
                // Calculate the unit price as the price per sq metre * the defined sq metre
                return Price.Calculate(price.Value * sqMetre, taxRate, currencyId);
            }
        }

        // No sq metre property found on the order line so use normal calculation process
        // which will ultimately fallback to using the variants `price` property
        return base.CalculateOrderLineUnitPrice(order, orderLine, currencyId, taxRate);
    }
}

Which you'll need to override the default implementation in a composer

[ComposeAfter(typeof(VendrWebComposer))]
public class Composer : IUserComposer
{
    public void Compose(Composition composition)
    {
        composition.RegisterUnique<IOrderLineCalculator, SqMetreOrderLineCalculator>();
    }
}

I've just written this stuff of the top of my head so I can't guarantee it'll work, but it should be the basic outline so you'll need to try it and debug anything that isn't quite right.

Matt