Not honoring written guarantee

A technician (Johannes) assisted us with the reparation of our fridge quite quickly, which was great. But now it's still not working properly. When I requested he'd be send back, (their invoice states they give you a 3 month guarantee), Cornel responded and said they don't offer that service and Johannes wasn't supposed to use the invoice. But fact is, he did. So I have a black on white invoice stating a three month guarantee which they now don't want to honor. They're also not answering my e-mails, probably hoping I'll just go away and they'll get away with poor service and \*****ing"our money"