Hsn code - services for marketing in india for foreign companies

Dear All,

I am representing some Foreign companies for Marketing their product in India & SAARC region. Kindly advise:

1. Has a GST INVOICE for entire commission  to be raised on finalisation of the buyer's PO and ADVANCE payment ? or ONLY on the ADVANCE payment ? The terms of my contract with my principals - foreign is that I can invoice based on payment schedule.
2. What will be the GST chargeable - % ?
3. What is the type of GST - CGST + SGST or IGST
4. What will be the HSN code - for "commission on sales"
5. For sales done in SAARC nations other than India what would the billing method to be used ? 

Thanking you all in the assistance that I await from you.

Regards,

Ashok Dias