What is notarization of contract? How much does one pay for the notarization of contract?

I have a company that exports handicrafts. I have three customers in China who have purchased the products. Contracts have been prepared covering terms of payment and executing the contract. They say the contracts would have to be notarized and that I have to send my part payment of the notarization fee to them to do the notarization in China. I asked them to take part of the amount due me to do the notarization but they said it can not be done that way. I need advise.