There is no official method on eBay to get additional monies, you are supposed to have the shipping set up correctly.
Rgardless, you have two options;
1 - cancel the sale and refund, adjust the listing so the shipping is correct and have the buyer repurchase.
2 - ask the buyer for their email address and send them a PayPal invoice (or use Venmo or similar)
I'm assuming this is about one of your large lots, is your buyer in Canada, the US or elsewhere? I noticed that the shipping to Canada was more than double the US shipping rate, that doesn't make sense.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.