It's perfectly acceptable for a seller to UPGRADE shipping and use a faster and more expensive method than was advertised. But sellers are NOT allowed to downgrade to a cheaper and slower shipping method if they've advertised priority mail.i wouldn't care, personally. i don't think you're entitled to a shipping refund, but could note in feedback that it was listed as priority but sent parcel post. it could also be an error on the seller's part. sometimes i do the opposite - list parcel post and ship priority. sometimes i list priority and ship something else. most people don't care, really. if someone really needs a package quickly they usually will mention it beforehand. most people are truly very accommodating and just happy to get what they ordered, myself included, lol
You say "most people don't care." I disagree. People expect to get what they paid for, whether it's the item or the shipping method. Why should the buyer have to wait several extra days because the item wasn't sent by the speedy service offered?