I don’t really think there’s a right way or a wrong way. Our rule of thumb is the parts of a solution that deal with ‘back office’ systems are done with Enterprise Server, and external facing components are done with Integration Server (for example, interacting with retailers and vendors).
With Enterprise Server your servlet could connect as a Broker Client and simply publish a document to a PeopleSoft adapter. This would be good if all you need to do is connect, send the invoice, and disconnect. Probably a rather easy solution, except for the learning curve of setting up the People Soft adapter. If the B2B can connect to People Soft with less set up or knowledge, you might want to take that approach. If for example it has some API or interface to accept XML. I have not worked with People Soft yet so my suggestions are somewhat limited.
One thing to think about though is if your servlet needs to wait for the invoice to fully process (some type of acknowledgement) before returning to the end user you’ll probably need to consider how much of a wait is too excessive.
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