There is no way through the ebay system, buyer should have asked before buying. 

Only way through ebay would be to cancel sale and relist with correct postage. As this refunds buyer then they buy again, but might be very in convenient.

Other way  is via paypal or bank transfer.

Listing has 2 postage options, standard & express, buyer should have chosen express when paying.

You can't invoice buyer extra cost.

You could cancel item & relist with next day postage or give buyer your paypal or bank details.

If you post item today, unless you pay premium for Saturday delivery, it won't get there until Monday.