How does third-party online bill pay organizations work?

Question: Third-Party online bill pay service (not banks) how do they send the ACH files, get the money from the customer, and notify that the bill has been paid? I need as much detail as possible for my Senior research paper for college, I am a finance major.

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A lot of times billpay companies do not sent ACH payments, but cut paper checks. Certain companies that support ACH payments can accept them. =Here are some sites to look at:

http://www.nacha.org/about/what_is_ach_.
http://www.cbp.gov/xp/cgov/import/operat...
http://www.ilovechecks.com/ach/howitwork...
http://www.ach-payments.com/how-ach-work...

More available at google :)

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