Taxes are just charged once on your prepaid pack order - at the time of purchase - NOT redemption. So when you purchase a prepaid pack, the appropriate taxes are charged to your purchase. When you redeem a pack, taxes will not be charged on the product.
Shipping (and the tax just on Shipping) is charged at the time of redemption. Normal shipping rules apply during redemption. For example - you will have to pay Rs. 84 for the 1st 50 4x6 prints and Rs.10 for every additional 10 prints thereafter. Service Tax is charged on the total shipping charges at the time the pack is redeemed.
You will never be charged twice or pay more for taxes. You will always pay the same rate for shipping as you would for a non-pack order.