Years of practice have led me to this practical wisdom:
As long as things are consistent, no one cares if you are correct. If you lose a penny in the backend calculation, and the frontend shows the amount without the penny, and the email contains the amount without the penny, and the PDF download contains the amount without the penny, no one will care that there should be a penny there.
It becomes problematic if some places are wrong and some are right, and they are not consistent. You won't get credit for being right in only some places.
As long as the error is small enough to be inconsequential, being consistent is more important than being correct.
As long as things are consistent, no one cares if you are correct. If you lose a penny in the backend calculation, and the frontend shows the amount without the penny, and the email contains the amount without the penny, and the PDF download contains the amount without the penny, no one will care that there should be a penny there.
It becomes problematic if some places are wrong and some are right, and they are not consistent. You won't get credit for being right in only some places.
As long as the error is small enough to be inconsequential, being consistent is more important than being correct.