Hmmm.... I don't think I have been clear. The more I work with this, the more obvious it is that this is a web development question instead of an e-junkie question, but here goes anyway:
Whether I am using custom or variations, I need the customer to type their account number into our web site, click the "Add to Cart" button, and have that account number sent to e-junkie, either as custom or a variation, or whatever. What I don't know how to do (although I think it is pretty basic....) is how to have the information that the customer inputs appended to the url that the button sends to.
In other words, I can easily hard-code a value for [%custom%] or [%on0%] into the button code, but then that value is the same for every customer. How would I make that value change for each customer? I'm thinking I'll need a form that, upon submission, transmits that data to a script that generates another web page with the button code generated with the info that was submitted.