When you get those "pre-approved" letters in the mail for everything from credit cards to pre-approved personal loans and similar type junk, do not throw away the return envelope.

Most of these come with postage-prepaid return envelopes, right?

It costs them more than the regular 10 Rupees postage "IF" and when they are returned. It costs them nothing if you throw them away! In that case, why not get rid of some of your other junk mail and put it in these cool little, postage-prepaid return envelopes.

Send an ad for your local dry cleaner to Citibank.
Send a pizza coupon to ICICI.