When and where to pay igst during export?

As per the provisions contained under IGST law, export of goods or services or both are to be regarded as “zero-rated supplies” and a person being a registered taxable person exporting such goods or services or both shall be allowed to claim the refund of the GST paid under one of the following two options: Export of goods or services or both under bond or letter of undertaking (LUT) without paying any Integrated Tax and can claim the refund of unutilized input credit. Export of goods and service or both on the payment of Integrated Tax and the exporter can claim the refund of the GST paid on such goods and services so exported. The above-mentioned refunds will be subject to certain rules, procedures, and safeguards as may be prescribed.

Yes, theorywise I know this. As I said before, we have not applied for LUT yet (and our CA is out of town). But I am clueless about two things:

We are about to dispatch a export shipment. So,

01. As we will be exporting with paying IGST, I don't know where to pay IGST to? should it be paid at airport customs while dispatch or should it be paid on GST portal?

02. And when sould it be paid? before dispatch or after dispatch or duing our next gst return?

You don't need to pay igst on export ,but you have to mention igst on shipping bill to claim refund. Please consult your CHA they will certainly guide you in this regard