We've had huge issues with the eBay postage calculator...so much so, that we now list almost all of our items as 'freight' and send the vast majority by freight, or use click & send, and put the actual shipping amount in our descriptions.  It's a huge PITA and would be so much easier if the shipping calculator actually did what it was supposed to, but it doesn't, so we've learned to adjust.

We realised that there was an issue when buyers started messaging us with indignant messages accusing us of charging exhorbitant shipping (which we most certainly didn't and don't BTW), and then we checked and were stunned to learn that the shipping calculator was way over estimating our shipping quotes.