what happens when the PO make a mistake and charge you for an item weighing 170g instead of 1.7kg?

Assuming you sent it - it will trundle through the system until someone or something notices it has not got the correct postage at which point it will get a surcharge slapped on it and the person at the other end will have to pay the surcharge before they get the item. The surcharge is more than the underpayment. You will then have all hell on to argue that it was their fault not yours although if it is a printed lable rather than stamps you are in with a better chance.

pff if you want something doing...
i'd had a quote for £4.50 from them so why my misses didn't query it...

ebay buyer just sent message saying its been delivered this morning