Well, if it's sent already, you can't.
No one is going to search through containers of parcels heading for North America, find your parcel, take it back to the seller, collect more postage and return it to the mail stream.
Which wouldn't speed it up at all.
Wait until the parcel arrives.
IF the seller did use a slower cheaper service than he advertised and you paid for, you can file an Item Not As Described dispute with eBay/Paypal.
Actually first you should contact the seller and ask for a refund of the difference in costs for the two services. If he refuses, move to the Dispute.
Again, when the parcel arrives, and your transaction with the seller is over, you can leave appropriate feedback. The most effective feedback is calm and factual. 'Charged for express, shipped surface. Late for Xmas & Easter.' for example.
At the same time you will be asked to leave Detailed Seller Ratings which are much more important to sellers. A normal transaction gets Five Stars. The lowest Rating is ONE Star. If a seller's rating drops below 4.3 there are heavy penalties for him.