How can I make an internal billing payment module?

[fyang]

Hello everyone,
I'm trying to apply Zencart for Internal use to manage our office inventory. Therefore, I need two main customization. First is to authenticate users with AD, I haven't decided on this, maybe not, I leave this to be done later. Second is to add a bill to module because there's no need to pay, users just need fill out a couple of fields (account number, organization number and project number). You may curious about shipping, well, I enabled free shipping (for inter-office mail) and store pick up which satisfy our needs.
I'm tackling the second part, if I can't implement it I'll have to give up Zencart unfortunately. I plan to put two dropdown lists on the check out page pulling from two new tables I added for accounts and organizations, and write them to the Order table (I've added two columns: account_num and org_num in Order table) in Order.php create() function.

Can someone give me some suggestions on this? I'm quite capable on coding. I didn't find an available module to do that, there maybe one out there. What should I put the dropdown list value in, session or this->info to reference them in Order->create?

The environment is Zencart 1.5.3 on Windows with php 5.3, Apache 2.2. I have a new install by zc-install, with Responsive-All_business template and Fast-and-Easy-Checkout module.

Thanks in advance,

[fyang]

It seems nobody have experience working on this. I've worked out adding bill to account numer, org number to Order table, adding these two numbers as dropdown list when check out and admin can update these two on orders. If anyone is interested at this, I might be of help.
Cheers,