Here's the thing.
There is absolutley NO WAY to change listing to charge MORE shipping than the listing states after you buy it. Once purchased, the listing is 'frozen' and nothing can be changed. We can revise an invoice and charge you less, not not more.
It was probably parcel post that the seller had included in the purchase price [what you call free - but isn't since there is no such thing as free postage], and when you paid, either ebay or paypal gave you the priority shipping option instead.
Of course ebay and paypal will deny it.
Paypal has an uncorrected glitch for over a year now that tells the sellers you paid for priority/insurance no matter what you paid for.
and ebay once did have a glitch where in the search feilds, EACH listing said shipping was free. Of course when you clicked on the listing, it didn't say that, but many buyers felt duped, and rightly so, but took it out on the sellers, and finally ebay offered to reimburse sellers the shipping cost if they refunded their buyers.
A nuetral is the same as a neg as far as eBay is concerned. So if you don't think the seller deserves a neg, and yer only 80% happy, then don't give them any FB at all.