It would be rather unusual if custom duty is charged on used stamps, even if those are coming from China. The letter must have been containing coins but I don't see any reason to charge custom duty on that unless there are large number of coins, making the letter very heavy.
The customs' department does ask for RBI license to import currency from other countries but believe me, you don't have to provide anything to them. Just write them an e-mail [you can find the e-mail address for the Ahmedabad customs on Google] saying that it's a gift from friend which you do not intend to resell it in India. If they insist to provide some value, just say £5 or so and they will then charge custom duty on it as per rules. If the coins were bought from a seller, the seller must have included an invoice inside so they will straightaway charge duty on it looking at the invoice and you won't even get a notice - you just have to pay the required amount to the postman.
When I first received similar letter from customs back in 2013, I was hell scared after reading all its demands. I was so tensed that I decided to visit them personally [they have a regional office in South Mumbai] where I met a very nice customs' officer named Mr. Gupta. He was friendly and listened to me patiently but ultimately ruled that I will have to pay the customs duty and it was in a sense justified as it was a purchase from a German dealer so not a gift definitely. The letter had Austrian €25 dated 2013, that's what caught their eyes apparently.
Since then, I have paid custom duty quite a few times, received similar notice couple of times. I only provide them the invoice through e-mail, never even mentioned other documents like RBI license etc. It works just fine.
Aditya