PayPal is preferred. Under certain conditions other types of payment services like FirePay.com and Neteller.com may be accepted. Please email and ask beforehand if you wish to use alternative means of payment.
You can specify which type you wish. Only if the finished translation is in Chinese do you need to specify. If you don't specify, one will be chosen for you. People in mainland China (Peoples Republic of China) use Simplifed Chinese, while Taiwan, Hong Kong, and Singapore use Traditional Chinese characters.
You can choose which type you wish. Usually you will receive the file back in the same format you sent it in. Normally, you send the work to be translated as an attachment by email. Common formats are html, Word 6 format, RTF format, and unicode text file.
The operating system (like Windows) needs to be configured to be able to display the language that you wish to read. If you try to read the document without the fonts and display support installed, the words look like garbled ascii characters, not Asian characters. Note that sometimes you need to reboot to the correct language display support in order to read different asian language documents. You can get around this by having the words encoded in Unicode.