AWK how to count sum

Save the sum to a file called sum, sorted

awk -F, '{a[$1]+=$3;}END{for (i in a)print i", "a[i];}' filename | sort > sum

cat sum 
product_a, 515
product_b, 348
product_c, 495
product_d, 27

Join the two files, first column of first file with first column of second (think "keys"); pipe it to awk and print reordered columns, using , as field separator (-F) and as the Output Field Separator (-OFS)

join -t ','  -1 1 -2 1 filename sum | awk -F, -OFS=, {'print $1,$4,$2,$3}'

product_a,  515,  domestic,  500
product_a,  515,  abroad,    15
product_b,  348,  domestic,  313
product_b,  348,  abroad,    35
product_c,  495,  domestic,  411
product_c,  495,  abroad,    84
product_d,  27,   domestic,  25
product_d,  27,   abroad,    2

Assuming that every line has the same number of fields (columns):

awk '
    NR==1   { for (i=1; i<=NF; i++) header[i]=$i; next; }
            { for (i=1; i<=NF; i++) total[i] += $i; }
    END     { for (i=1; i<=NF; i++) print header[i] "=" total[i]+0 }
    '

Sample input:

red green blue
1 2 4
8 16 32
64 128 256

Corresponding output:

red=73
green=146
blue=292