If you pay at the beginning of the month, the balance on which interest is charged is $250 less than it would be if you paid at the end of the month. The difference is 1 month's interest on $250.
... I = P·r·t
... I = $250·0.1375·1/12
... I = $2.86
The difference in interest paid is $2.86 if you pay at the beginning of the month instead of the end.
