Abstract PurchaseService for IAPs

PurchaseService.cs

//NOTE: to be used as a singleton in your app
public abstract class PurchaseService
{
    /// <summary>
    /// Retrieves the prices from iTunes or Google Play
    /// </summary>
    public abstract Task<Purchase[]> GetPrices(params string[] ids);

    /// <summary>
    /// Buys an in-app purchase
    /// </summary>
    public async Task Buy(Purchase purchase)
    {
        var receipt = await BuyNative(purchase);

        //TODO: validate from our server here
    }

    protected abstract Task<Receipt> BuyNative(Purchase purchase);
}