On one of the unpaid listings, use the Contact Seller link to , well, contact the seller.
Explain your error, politely, and ask if he can send you a Mutual Cancellation request.
If he has already opened an Unpaid Item Dispute, he will not be able to.
The MC allows the transaction to close amicably and both members can leave feedback.
A UID however is more serious. If the seller allows it to close by itself, without taking further action, he removes his listing from eBay for several days, possibly losing an actual paying customer.
If he does not pursue the UID, he has to pay eBay fees, upwards of 10% of the expected payment, for YOUR error.
If he continues with the UID, he gets those fees back, and you get a Strike for each of those unpaid items. No feedback can be left.