Looking for a way to be able to have a lookup form in the checkout payment section

I'm working on a Drupal Commerce website that allows users to use a few different forms of payment. One method is a custom external method and I am currently able to query a user's account balance in the Account section, but I'd like to offer customers the chance to lookup their account balance during checkout. Since the entire checkout area is a large form I'm running into issues trying to actually submit something. I'd rather not get into using too much crazy Javascript such as pop up divs but I may not have a choice. I'm wondering if there is another obvious route that I could go that would work? Thanks!