If the parcel is accepted at your post office, or wherever you send from, the buyer may have to pay more at their end. It's impossible to say for sure. Ideally it's best to cancel/refund the transaction with buyer's agreement and relist with correct postage.
To choose your own postage for large items, during the listing process click on the box under the heading Package Size. At the very bottom of the pop-up you should see Oversized. Select that then you will be able to buy your own postage (it will say SD is unavailable for this item). It has to be done on a laptop or desktop rather than the app.
If the option doesn't show up try changing sub-category or category. As you're relisting for a specific buyer the category you use won't be vital.
If the buyer doesn't want to pay more postage you'll have to cancel and relist.
An alternative to the above is to list with free postage and factor that cost into your item price. And buy your own postage outside of ebay. To get the buyer's address and mark dispatched you can open the original prepaid label (Get prepaid label > Get prepaid label and QR code). The option to change the tracking has been removed but you would be able to add the new number to a "not received" case if that were necessary.
You can claim a refund for an unused label here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
Without tracking on ebay the payment may be held for 14 days.
