Google Sheets – Calculating Due Date for a Bill

Assuming you have "BANK" in I3:I20 (as opposed to "AMEX") for bank payments, then I believe you can achieve your goal by reversing your logic (equal to "BANK", rather than not equal to "AMEX")...

Change the formula in F24 to:

=arrayformula(sumif(H3:H20 & I3:I20,E24:E33 & "BANK",D3:D20))

This will then populate cells F24:F33 with the appropriate values. & is the string concatenation operator, so this effectively compares "1BANK" with "1AMEX" (for instance).

I wrote a custom function in Google Apps Script that takes all the criteria you want it to have (and more) in consideration.

Code

function myPayments(paymentPlan, dateRange) {
  var output = [];  
  for(var i=0, iLen=dateRange.length; i<iLen; i++) {
    var checkDate = new Date(dateRange[i]);
    var weekDay = checkDate.getDay()+1, sum=0;
    for(var j=0, jLen=paymentPlan.length; j<jLen; j++) {
      if(paymentPlan[j][1] == weekDay) {   
        if(paymentPlan[j][2] === "" && paymentPlan[j][3] === "") {
          sum += paymentPlan[j][0];
        } else if(paymentPlan[j][2] === "") {
          if(checkDate <= new Date(paymentPlan[j][3])) {
            sum += paymentPlan[j][0];
          }
        } else if(paymentPlan[j][3] === "") {
          if(checkDate >= new Date(paymentPlan[j][2])) {
            sum += paymentPlan[j][0];
          }
        } else if(paymentPlan[j][2] !== "" && paymentPlan[j][3] !== "") {
          if(checkDate >= new Date(paymentPlan[j][2]) && 
            checkDate <= new Date(paymentPlan[j][3])) {
            sum += paymentPlan[j][0];
          }
        } 
      }
    }
    if(sum == 0) {
      output.push([]);
    } else {
      output.push([sum]);
    }
  } 
  return output;
}

Screenshot

Remarks

If you extend the paymentPlan range, to contain the rest of the data as presented in the PAYMENT PLAN sheet, then you will notice that overlapping payments will be summed.

Add the script under Tools>Script editor and press the save button (in your case it's already there).

Example

I've added the solution in your example file.