Harry, if you mean me, I have paid with credit (not debit) card as well as POLI (bank transfer) previously. In both cases the payment is made after all the flight details (including fringe add-ons) have been finalised.
And I have just tried out SQ's online booking, and the passenger details section does not ask for passport information other than passenger name. So I don't see how PAXboy can reasonably expect SQ to inform him of passport requirements.