Dependency injection w. multiple implementations | Mario Codes

I have the class package with a type, plus more info not relevant to the question

public class Package
{
	public string Type { get; set; }
	// ... more package properties
}

I have the following interface service to mail packages

public interface IMailService
{
	public Task<string> SendPackage(Package package);
}

Then I have several implementations for this interface

public class MailNewPackageService : IMailService
{
	public async Task<string> SendPackage(Package package)
	{
		Console.WriteLine("send package through new service");
		// implementation ...
	}
}

public class MailOldPackageService : IMailService
{
	public async Task<string> SendPackage(Package package)
	{
		Console.WriteLine("send package through old service");
		// implementation ...
	}
}

I need a delegate which takes a string and returns a IMailService

public delegate IMailService MailServiceResolver(string key);

At the startup we set the service with a key

// we define all the implementations
services.AddSingleton<MailNewPackageService>();
services.AddSingleton<MailOldPackageService>();

// we set the resolver to return a service based on a key
services.AddSingleton<MailServiceResolver>(provider => key => 
{
	switch(key)
	{
		case "new": return provider.GetService<MailNewPackageService>();
		case "old": return provider.GetService<MailOldPackageService>();
		default: throw new KeyNotFoundException();
	}
})

We set the consumer

public class UpsOffice : IUpsOffice
{
	private readonly IMailService _mailNewService;
	private readonly IMailService _mailOldService;
	
	public class UpsOffice(MailServiceResolver resolver) 
	{
		_mailNewService = resolver("new");
		_mailOldService = resolver("old");
	}

	public async Task<string> ProcessPackage(Package package) 
	{
		// ... conditionals to check which service to use
		if(x == true) 
		{
			// ... processing through new service
			_mailNewService.SendPackage(package);
		} else 
		{
			// ... processing through old service
			_mailOldService.SendPackage(package)
		}
	}
}