First contact the seller(s).
You don't do that here. This is a member to member advice and chat board.
The seller can cancel the unwanted items. This may involve a Request for Mutual Cancellation which requires you to actively accept the Request.
The cancellation releases both of you from the Transaction. Both can leave feedback. "Thank you for your help with my error" would be appropriate.
If you don't do this, the unhappy seller may open Unpaid Item Disputes on each listing.
If you do not pay, you get a Strike against your account.
Most seller have automatic Blocks against bidders with two or more Strikes, so bidding on eBay will be much more difficult.