How do you explain that people don’t send mail on Erev Shabbos?
What people? I’ve never heard of such people and doubt they exist. Maybe you don’t do this, but you are only “person”, not “people”.
And as Daasyochid pointed out, even if you are guaranteed Saturday delivery that is not necessarily Shabbos. It’s the nochri’s choice whether to make the delivery on Shabbos or after, so there is no problem at all.