GST related to supply

It depends on the billing address, if the billing address of customer is Delhi then IGST will be applicable

As per Rule 46(1)(n) while issuing Tax Invoice it is mandatory to put Place of supply along with State Code & same was also clarify by Circular 90 GST the consequences of Non compliance of not quoting the state code is initiation of penal provsion under Section 122 or 125.

As per Rule 138 e-way bill is required to be generated if value consignment is >50 k & being transported via motorized conveyance. So even though goods being handed over at Mumbai the eway is mandatory.

So once you put the state code you the place of supply will be Delhi Hence IGST will applicable

If its a bill to ship to transaction, then the place of supply would be the billing address i.e., Delhi GSTIN and you will be required to charge IGST.