How about
"Hi I have received the item but I am a little unhappy with the way I have received it. First I paid $x (x because I cant be bothered to go back and check) to have it shipped and you've stuck it in a padded envelope. I feel I have been over charged for the P&P costs. Second the slab itself while not cracked or scratched has arrived really dirty. Like really, I even think there might be a booger on it. Is there anything that can be done regarding the p&p costs or the price on the slab due to the dirty condition it has arrived in? Thanks for taking the time to review my message, I hope to hear from you soon."
If they reply positively then great. Work from there. If not thank them for the reply and leave the feedback.
I mean it cant be that surprising to hear "hey, maybe speak to the seller first". At least give the person a chance. If I was the seller I would throw some money back to you to try and secure the positive feedback, they may do the same.