Message the buyer now, before you decide anything. A seller who says "I have broken it, here are your options" on day one keeps most of the goodwill. One who goes quiet and cancels on day five does not.
Your three options
1. Offer it at a reduced price, honestly described. Photograph the damage and let them choose. Works more often than you would think, especially for something hard to find.
2. Offer a replacement, if you have another.
3. Cancel and refund. Clean, and the right answer when the item is ruined or they want out.
Let the buyer pick. It is their order.
What cancelling costs
It counts against you. Seller cancellation rate is one of six measured standards and has the tightest thresholds of any of them. One is recoverable; a habit is not.
No fee is earned, because a fully refunded order is not a sale. So the loss is the item and the sale, not a charge on top. The seller cancelled my order
Cancel early rather than late. A cancellation on day one is forgivable. On day six, after the buyer has waited, it is the review you cannot undo.
What not to do
Do not send it broken and hope. It comes back as not as described, within 7 days whatever your returns policy says, and you lose the postage both ways as well as the sale.
Do not repair it and say nothing. If it is visible, it is a misdescription. If you do repair it, say so and reprice.
Do not blame the courier. It had not been posted.
Then fix the packing
One breakage is bad luck. Two is a method problem.
Did the item touch the box? Anything fragile needs padding on every side, including underneath.
Was the box the right size? An oversized box lets things move, and costs more to send anyway. Why does a big light box cost so much to send?
Did it break while being packed, or while being handled? Different fixes — the first is your bench, the second is your materials.
How to pack an order · Fragile item delivery
For anything genuinely fragile
Offer pickup as well. No courier, no box, no risk — and for pottery, glass and framed work it is often what the buyer would prefer anyway. How to offer pickup
Price delivery for the packing it actually needs. Proper materials cost money, and a rate set for a padded envelope will not cover a double-boxed vase. How do I work out what to charge for postage?
More: how to pack an order, how refunds work, what packaging should I buy.