If my employer sends a laptop by mail from the USA to India, Is there any customs duty that will be applicable to it?

Avoid it, they'll be paying hefty shipping already and you'll pay import duties on it, which may end up making total cost much higher than buying same laptop in India. Although there are already established rules on how much to charge for what imported item, Indian customs is notorious for being very arbitrary in determining ransom to release your item.

unless they're sending you a machine w/ keylogger it is easier that you purchase here and send the invoice

Yes. You will be charged customs. Alternatively, some services like Bluedart can pre-charge approximate customs and will refund to your company if the customs charged are lesser. Like everyone else says, not worth it. Just buy a laptop from here and get it reimbursed. Or even they can buy locally and ship it to you.

My laptop was stuck at customs for some clarification. I was able to get a letter from my employer stating the laptop belongs to my employer and it is being provided for work. Sent the letter and got the laptop soon after.

thanks for the inputs guys, sure will do accordingly