Wrong Package invoiced and debited for

On 12 May (around 1pm) I upgraded to a higher package. On 13 May I received my invoice dated 11 May 2015 showing a charge for the higher, more expensive package. After 2 emails to MTN I eventually went to the Killarney branch where upgrade was done, they assisted with logging a query (27 or 28 May). MTN then debited me with the wrong higher amount. I finally received a call back from MTN on 4 June. The lady, Glenda, admitted that nothing has been done regarding the query due to the strike action, but escalated it to a supervisor. I received a call back later in the day from Glenda stating that the amount billed for was correct as the account only closes at 12 am on the 12 of May (note that the invoice is dated 11 May). She said I need to take this up with the Killarney branch as they did the upgrade and should have explained the account closing on the 12th only (this was not done). <br> Even though I had the benefit of the lower tier package, I have now been charged for the new higher package, even though my account and invoice is dated the day before I actually did the upgrade. MTN refuses to credit or refund the difference and also has not contacted me again as requested.