I've noticed that SM will generally distribute the first payment in given month to cover the minimums they calculated for each loan account. Like you, I like to pay beyond the monthly minimum so with any additional amount beyond the mininum, it automatically asks me how I would like to distribute the remainder, i.e. if my minimum payment is $250 and I choose to pay $300, then they will distribute the first $250 how they see fit to cover the minimums, then I can choose any combination of the individual loan accounts for the remaining $50 to get distributed into. Strange that you can't do that, I figured it was a pretty standard online payment option.