Actually, the buyer was rather generous counting $2 for the envelope.  last time I looked the padded bag in that size was around $1 (may have been 80c). 

Talking about jumping into conclusion; how do you know the buyer did not ask about the grossly inflated postage charge before leaving the FB?

If AP staff try to charge people parcel rate for anything less than 2cm thick they are ripping them off.  They tried it on me few times, and when I told them that anything under 2cm entered into the parcel section of the AP calculator gets automatically redirected into large letter section, they sheepishly charged me the correct rate.  There is no excuse to overcharge buyers just because the PO staff might try to charge you wrongly.

According to eBay sellers suppose to charge only the actual cost of postage, packaging materials plus they are allowed to add reasonable handling costs.  That is mainly for large sellers who actually employ people to do the dispatch, and then take it to the PO in bulk, thus dividing the cost of handling between 50, or hundred or whatever items.  Not for person who sells one book and takes it to the PO.  If somebody lives 100km from the nearest PO would you expect them to add the cost of the petrol for the 200km trip to one buyer?  No, of-course not, the seller in this situation has to work out how to post when they already go to the town for something else.