In my limited experience, I have discovered that a lot of buyers don't read ads. They just look at the photos. I have a paragraph in my listings requesting them to wait for an invoice if they have purchased more than one item that incurs postage costs, otherwise they'll pay too much. Still, I get people bidding on multiple items, then paying for them as they win them. I used to issue a refund for the difference in postage, but now I just send them as separate items as I don't want to risk getting any more defects.
It is up to you as the seller to make sure the postage quoted is correct. I have bought a few things from the US and UK where the postage I was charged was far less than what they had to pay. I used to offer to send them the difference, but they would always say no as it was their error and they had to wear it.
I nearly got stung myself just yesterday. I advertise large letter items as free postage and parcels, the buyer pays the postage. Yesterday I packaged up an item into a padded bag, that had sold with free postage and when I went to check it through the 20mm gauge, it was a right fit, but still went through. Normally I would have just put it in the red box and hoped for the best, but this particular buyer wanted it sent registered, so it had to go inside. I crossed my fingers and hoped for the best. The lady said that it was a tight fit and probably should go at parcel rate, but she let it through for the $1.40 that I had already stuck on it. It seems that when I measured the item prior to listing, I didn't look at the tape measure properly and it was a few mm thicker than I realised. I could quite easily have been charged $7.20 to post that item, which sold for $7.99. I would've had to wear the costs, and the loss, because I didn't calulate the postage correctly. If a seller upped the postage after I had won/bought an item, they would end up with a red dot and a low DSR for their efforts.
