handler.cs

public class ChangeCustomerPricingLevel : IAsyncRequest
{
    public Guid CustomerId { get; }
    public PricingLevel PricingLevel { get; }

    public ChangeCustomerPricingLevel(Guid customerId, PricingLevel pricingLevel)
    {
        CustomerId = customerId;
        PricingLevel = pricingLevel;
    }
}

public class ChangeCustomerPricingLevelHandler : IAsyncRequestHandler<Envelope<ChangeCustomerPricingLevel>, Unit>
{
    private readonly CustomeRepository _repository;

    public ChangeCustomerPricingLevelHandler(CustomeRepository repository)
    {
        _repository = repository;
    }

    public async Task<Unit> Handle(Envelope<ChangeCustomerPricingLevel> message)
    {
        var customer = _repository.Get(message.Body.CustomerId);
        try
        {
            customer.SetOperation(message.MessageId);
        }
        catch (InvalidOperationException)
        {
            return Unit.Value;
        }

        customer.ChangePricingLevel(message.Body.PricingLevel);
        _repository.Save(customer);

        return Unit.Value;
    }
}