Ebay postage calculator completely different price to real postage

[kylielam2005]

I did the Ebay postage calculator & it said $12.75, but when I went to post the item at the Post Office it was $88.00!! I re-checked the Ebay one & it was still really cheap (& did a screen shot), but the Australia Post online one said $88.00.

I'm a new seller, I also offered the purchaser a full refund after explaining what happened but she doesn't believe me & keeps asking for the "correct" postage price.

I even sent her a screenshot 3 times (also a screenshot of the error).

What can I do? Obviously I wouldn't have listed the item if I knew the postage was that high? And why did it give me such a cheap price? I entered the weight, box size, ticked "box" etc etc.

Thanks.