is the payment due the 14th, 15th

Taxpayer is filing a tax return where he owes like 50k.

I still have to prepare the return.

Do i need to file and remit payment by the 14th or 15th to avoid
another month of late payment and interest?

Yes, file and pay by the 15th to avoid another month of penalties.
Interest is calculated on a daily basis, so if you file after the 15th, even though you will incur another month of penalty, interest will only calculated based on the day of payment.

do you think if I just pay the whole amount by the IRS website on the 15th i'll be ok and avoid any addl late payment penalty?

client would prefer a few days to get to review over the paperwork

ty!