Welcome to Upwork Marie,
We have two types of contracts, fixed price and hourly contracts. In order to be eligible for ourUpwork Payment Protection you will need to be hired on a official contract and fulfill the criteria required.
Your clients will have an invoice created for each transaction, there's no need to send any additional bills.
Regardings payments (withdrawals), to learn more about our accepted payment methods and how to add one check out this Help Article. I would also suggest checking out the resources on this Thread as well.
If you have any additional questions let me know, thank you.