Removed redundant functions from Service::class

app/Models/Invoice.php

@@ -45,7 +45,18 @@ class Invoice extends Model implements IDs
 	protected $dates = ['date_orig','due_date'];
 	public $dateFormat = 'U';
 
+	public const BILL_WEEKLY = 0;
+	public const BILL_MONTHLY = 1;
+	public const BILL_QUARTERLY = 2;
+	public const BILL_SEMI_YEARLY = 3;
+	public const BILL_YEARLY = 4;
+	public const BILL_TWOYEARS = 5;
+	public const BILL_THREEYEARS = 6;
+	public const BILL_FOURYEARS = 7;
+	public const BILL_FIVEYEARS = 8;
+
 	/* Our available billing periods */
+	// @todo change this to a function - with billing_name()?
 	public const billing_periods = [
 			0 => [
 				'name' => 'Weekly',