GST EXPORT RELEATED QUERY

Query 1:-exporter raises invoices in advance and exports the goods at a later period. In such cases, value declared in invoice and the invoice value mentioned in the shipping bill may vary due to change in exchange rate. In such cases, for the purpose of GST return filing which value is adopted as the taxable value?
Query 2:- in case of zero rated supply there is no tax, then while filling the GST return, which value is considered as invoice value, it is equal to taxable value?