AS/400 DB to Constant Contact DB

We use constant contact to send out emails to customers. We used to do this via our AS/400. Here is my issue:

We not have and iPad set up on our Retail floor to collect emails from customers. As good as this may be our AS/400 has no way of knowing what emails may have been entered into the iPad info that is going to constant contact.. How can I make this work? I guess that is my question.

Is there a way for our AS/400 DB to query Constant Contact or vise versa?

The way I can envision is the cashier saying "Can i Verify we have our correct email address in our system" : Customer, yes it is aaabbbcc@mail.com, ok thanks. We do have that in our system)

Another way cashier saying "Can i Verify we have our correct email address in our system" : Customer, yes it is aaabbbcc@mail.com, ok thanks. We do not have that in our system can you enter in you info on the iPad so that we can have the correct information. Customer : Yes.)

Customer then enters their email address into the iPad screen the data then appears in realtime on the cashiers screen (AS/400 green screen) Cashier says : Thank you for that information. I now see this information..

Something like this.

Thanks all