Is OK polite? How to express that I've understood your words?

I'm not sure how to express that you have understood others' words.From this April I've been working in a bank. Occasionally there will be some foreign customers come. (Actually the frequency was about 2~3 people averagely a week last month, but this month I haven't see one yet.) Most of them just want to exchange some RMB Yuan (Chinese Yuan? Which one is used more often?) from dollars or euros. I use my poor English to ask them what they want to do and tell them the documents we need. But I find that I don't know how to express that I have understood their words.  Like following scene: I(behind the counter): What can I do for you? Foreign customer: I want to exchange my dollars into Chinese Yuan. I: OK. Like the above, I always say "OK". Maybe sometimes with "I see". But I don't know if these are polite and proper? or if they are polite and proper enough? What would you native speakers say in your daily conversations?  And what would a native bank staff, or say, a native waiter/waitress or other people work in service industry say in this situation? Thank you very much.

我想知道，回应对方时用的“好的”“我明白了”，在英语中怎么说？怎么说比较有礼貌（在服务业）？