Follow Ebay's guidelines.
Normally, if the buyer is the person who purchased the insurance (as happens with some auctions), you'd send them the slip, and they'd collect the insurance at the post office. The post office gets the item.
However, this is Ebay, and you purchased the insurance. Ebay's current policy is that the buyer returns the item to the seller at the buyer's expense. You issue the buyer a full refund. You then take the item to the post office to collect the insurance, which you purchased to cover yourself. The post office will keep the item. Ebay and the buyer don't care that you have insurance. They just care that you didn't safely deliver the purchase to the buyer.
The people screwed in this scenario are (1) the buyer, since they get nothing, and are the ones out the return shipping they have to pay for your poor packing and shipping, and (2) the post office, since they have to now cough up the funds to cover your poor packing and shipping.
Yes, you have to refund the buyer. They didn't get what they ordered. Legally, until the item is acceptably delivered to them, it's in your possession and your responsibility. If you didn't pack it right, or choose an appropriate shipping service, that's your problem. If you're honorable, you should even refund them their return shipping. If you don't do that, you should expect negative feedback, despite Ebay's policy. Ebay does allow for you to get the item back as a condition of the refund. It's up to you from there if you can resell it or any part of it, or if you purchased insurance on which you can collect, etc.
So....
(1) Hang onto the slip. Potentially contact USPS to see if they'd like the buyer to take the item to their post office rather than ship it back to you. USPS may well say they'll just hang onto the item at the buyer's end, and you can take the insurance slip to your own post office.
(2) Otherwise agree to issue a full refund upon return of the item. If you're honorable, offer to refund the return shipping that Ebay will otherwise force the buyer to pay.
(3) Issue the refund when the item is delivered. Ebay will take it from you if you don't do it voluntarily.
(4) Take the item to the post office for the insurance if USPS said the buyer should simply return it to you. Refund the buyer no matter what. It's your problem if you don't collect the insurance, perhaps because USPS says you didn't pack things right.