Export of GOODS the exchange rate on shipping bill is taken from Custom (Published on weekly basis) . Refer Section 14 of custom act .
> On export of SERVICES , Generally Exporter takes the Exchange rate of RBI . But As there is no hard & fast rule in opting exchange rate of RBI only , So you can also take Custom rate of Exchange for Export of Services, The difference can adjusted Under Head "FLUCTUATION IN EXCHANGE" Note : Many small exporter of Services also take the Bank Exchange Rate also.