Integration/syncronization issues

Hi all Traditionally we've been an "internet only" development company, developing sites without a need for integration with internal systems. Occassionally we have done this, for example a daily 500 record CSV upload for one client. However, this potential job we've been asked to quote for is much more in-depth with regard to integrations and potential problems. This is what I've been asked to do: 1. Develop a site which has a userbase of 4000 users, each of which has between 1 and 200 invoices 2. Enable this so that the users can come and change their account information, view their current status, or pay one (or more) of the above invoices 3. Provide a link with the internal accounting system so that transactions conducted on the web are added to their system at the end of the day. 4. Provide syncronization with the web database so that any off-line transactions are included in the online... There are roughly 12,000 invoices between these 4000 users. I have been told not to use CSV (by some other programmers) as it is too "flaky" (their words, not mine!) and I'm at a loss as to how to proceed. Currently we are using PHP and mysql and the clients internal system is using something called JD Edwards (I've been to the JD Edwards site and can't find a demo). Does anyone have any experience of this kind of syncronization, the potential problems and possible solutions? Some pointing in the right direction would help :) All the best Ade