Double charging customers

I used this courier services last year in July, and even paid before the services was rendered. Today, they sent my whole company an email saying that I still have not paid for it, and should pay as soon as possible. Luckily I still had my emails with the proof of payment in my sent items, so could resend it to her and the people I've sent previously in the courier company.<br> <br> None of them even sent me an apology. I requested it from accounts admin who sent me the mail this morning, but she refused to.<br> <br> I will never use their services again and I've got a feeling they double charge many of they customers like that, luckily I still had my proof my payment!