handler.cs

public class ChangePricingLevelHandler : IAsyncRequestHandler<ChangeCustomerPricingLevel, Unit>
{
    private CustomerContext _db;

    public ChangePricingLevelHandler(CustomerContext db)
    {
        _db = db;
    }

    public async Task<Unit> Handle(ChangeCustomerPricingLevel message)
    {
        var customer = _db.Customers.Single(x => x.CustomerId == message.CustomerId);
        customer.PrivingLevel = message.PricingLevel;
        await _db.SaveChangesAsync();

        return Unit.Value;
    }
}