If you don't have many customers ordering on your e-Commerce platform, I suppose you could set up a Temp field in VI to read the email address. Then you could set up as many "Customer Number" fields as needed, and set selection criteria to read the email address and assign the correct customer number. If you have many customers it could be really clunky, though, and I'm not sure if there's a limit to how many selection criterias (sp? criterium? ) you can use in 1 VI job.

Untested Idea:

VI to a generic customer number, then do a script on the SO header table (post-validate on the email address) to query the customer table for that value, then override the CustomerNo at that point.  VI results wouldn't likely show the results of the scripted customer # change, but it might work.

Or hire a Master Developer to come up with some Perform logic to do a lookup...