Should I give a rebate, how much is appropriate?

The rebate is the seller from the buyer to pay for the goods in a certain percentage of the price returned to the buyer. If you are a friend, you don't need to say that it is a rebate, because they are willing to help you, but also do not want to benefit from it, you just need to buy some gifts to thank them for the old can be. But if it is not particularly good, it can be appropriate to give a little, preferably in the form of red packets to give, generally 5% -8% of the profit is more appropriate to show appreciation, or to the same number of shopping cards can also be, is.