Either of them: A or B Both of them: A and B The total cost would be different. If you say both, the person who is talking to you may say, "No, no. I just need one", thinking , "What a pushy salesman!" or "How careless about cost efficiency!" It's quite a big difference, isn't it?

In this case "either" is like "any". If you can produce "either of them", that means you can produce any part of what is being asked of you. One or all. "Both of them" used in this sentence would mean that you would have to produce all that is being asked of you. Hope this helps.

Well, "Either of A and B" means "One (A) OR the other (B) BUT not all (A and B)". So, "Either of them" means "One OR the other BUT not all". But "Both of A and B" means "All (A and B)". So, "Both of them" means "All of them". And there's a big difference, right ?