Export sales Gst rate calculate @12% instead of @18%

You may need to pay additional 6%. That will become your cost since you won't be able to claim refund on it nor charge your customer.

yes agree with CA altamush Zafar

Deposit the balance tax with interest, issue the debit note & report it in your returns by giving reference of shipping bill.

(Can try for refund but as ICEGATE will not allow you to do the same on shipping bill you have already claimed the refund)