Calculate total copies of each book you want. Arrive at the total cost. Add Rs 40 + Rs 10 for each copy of book selected as postal charge.
For example: You choose to buy,
1 copy of An introduction to GCC, 1 copy of An introduction to Python & 1 copy of Comparing and Merging Files with GNU diff and patch, that would sum up to Rs 630.
You need to add Rs 40 + Rs 10 X 3 for each book = Rs 70 as postal charge.
So the total charge will be Rs 700 for all three books.