Hi what is the correct way to do this.

I create my note and save it:

var noteService = MerchelloContext.Current.Services.NoteService;
var note = noteService.CreateNote(invoice.Key, EntityType.Invoice, "My note text");
note.Author = "System Note";
note.CreateDate = DateTime.UtcNow;
note.InternalOnly = false;
noteService.Save(note);

But then I need to update the invoice notes collection - or do I - this part just feels weird.

// TODO: must be a better way
var allNotes = new List<INote>();  // new notes list
if (invoice.Notes != null)
    allNotes.AddRange(invoice.Notes); // add existing notes to new list
allNotes.Add(note);  // add new note to list
invoice.Notes = allNotes;  // pass new list to invoice
MerchelloContext.Current.Services.InvoiceService.Save(invoice);  // save invoice

Thanks