Sum the values in a column except the header

awk '{s+=$3}END{print s}' yourfile

Pretty trivial using just awk. Assuming the example data is in a file, ex.txt:

$ awk '{total = total + int($3)}END{print "Total Amount collected = "total}' ex.txt

Example

$ awk '{total = total + $3}END{print "Total Amount collected = "total}' ex.txt 
Total Amount collected = 95657

Details

Using awk we collect the values from the 3rd column ($3) and accumulate their sub-total in the variable total. Once complete, as the last thing to do, END{..}, we print the message along with the value of the variable total.

total=0; 
for n in  $( tail -n +4 /tmp/reports.txt | awk '{print $3}') ; 
do 
   total=$( expr $total + $n ); 
done ; 
echo ">>$total"