Almost every standard mortgage payment is due on the 1st of the month and has a 15 day grace period before a late charge.
If you mail a check, you avoid any extra fees.
Contrary to what many people think, there is no benefit to paying a monthly mortgage payment early in the month. It does not improve your credit score nor does it pay off your loan any faster.
The company you send your payments to is a SERVICER, they don’t own your loan.
Each one has its own rules about paying online and whether or not they charge an additional fee after a certain date in the month.
They CANNOT charge anything extra if you send a check that is received by the 15th.