Indenting agent - point of taxation

Sir,

I am indenting agent and receiving commission on sales of goods. I issue invoice to overseas principal once every quarter - my question is when is GST payable?

example

Sales during Jan - March 2021 : Invoice to be raised during April 2021.  Payment received from overseas supplier in April 2021.

GST payable: During May 2021

Is this correct?

or

Should I issue invoice every month even though my principal remits payment only once after end of each quarter?

Thank you.