If your (US or UK) seller is using the Global Shipping Program, he actually can't discount your shipping. He probably did not know that when he made that promise.
If this is the problem, suggest that he rebill you directly through Paypal's Send Money.
He can adjust shipping there.
PP has strong Buyer Protection so you can be confident in paying the bill.
And yes, be careful in the wording of your message. Email is very easy to misunderstand. You both want this transaction to be friendly and successful.
Politely. -- Captain Malcolm Reynolds