Ahh, ok, so you want to apply the remainder of the 1 year license cost and pro-rata discount the new license fee.

Then yea, I think this is a good candidate for a price adjustment.

Here's a bit of an outline of what I think you might need:

public class LicenseProRataDiscountAdjuster : PriceAdjusterBase
{
    private readonly ILicenseService _licenseService;

    public LicenseProRataDiscountAdjuster(ILicenseService licenseService)
    {
        _licenseService = licenseService;
    }

    public override void ApplyPriceAdjustments(PriceAdjusterArgs args)
    {
        // Check the order to see if we have a license upgrade
        foreach(var orderLine in args.Order.OrderLines) 
        {
            if (orderLine.Sku == "LIC-UPG") // Check to see if it's a license upgrade
            {
                var licenseId = orderLine.Properties["licenseId"];
                var remainingPeriod = _licenseService.GetRemainingLicensePeriod(licenseId);
                if (remainingPeriod > 0) 
                {
                    var price = new Price((10 * remainingPeriod) * -1, (0.1 & remainingPeriod) * -1, args.Order.CurrencyId); // Calculate discount price based on remaining period
                    var adjustment = new LicenseProRataDiscountAdjustment("Pro Rata Discount", "LIC-0001", price);

                    args.OrderLines[orderLine.Id].TotalPriceAdjustments.Add(adjustment);
                }
            }
        }
    }
}

[Serializable]
public class LicenseProRataDiscountAdjustment : PriceAdjustment<LicenseProRataDiscountAdjustment>
{
    public string LicenseId { get; set; }

    // A parameterless constructor is required for cloning
    public LicenseProRataDiscountAdjustment()
        : base()
    { }

    // Additional helper constructors
    public LicenseProRataDiscountAdjustment (string name, string licenseId, Price adjustment)
        : base(name, adjustment)
    {
        LicenseId = licenseId;
    }
}

Ultimately you need to create an adjustment type, then in your adjuster, you do your logic to work out if an adjustment applies. Here's I've assumed you've wrapped that in some kind of service that has been registered with the DI container and injected in your adjuster constructor.

In the ApplyPriceAdjustments method then, you look through the supplied order and find any order lines that qualify to be checked, then you use your service to look up the remainder of the license amount (here I've assumed you have added the licenseId as an order line property to the orderline). If the response from your service shows a discount is needed, then an adjustment is created and applied to the order lines total price.

You'll then need some logic on order finalization to cancel the other license and create your new one, but I'm guessing you'll be ok with that.

Hope this helps

Matt

Hey Johannes

Once you've created your price adjuster and registered it with the DI container, it should get picked up and applied automatically.

All price adjusters are run whenever an order is re-calculated, which is usually whenever something happens to the order that could affect the price, such as adding order items to it, or setting the shipping / payment countries etc.

// Matt