HI Scott. Welcome to the wonderful world of InfoPath.
You need to get the template, the .xsn file that the sales man would use to create a new form.
Right click on it and open it in Design mode
Go to tools and data connections.
In the list should be the one that submits to email.
Select it and select modify. It is possible they had a seperate submit for each part of the alphabet you might have to check this.
You will come to where it shows the to and cc and subject fields. To the right of subject will be a formula button "FX"
Use this and then you can add a concat("Request for Credit Approval for ", customername) assuming customername is the name of your field. Use the add a field button to select it.
If there are three then you should make one and then go into your submit button or rules and remove the logic to use three and have it use the one all the time.
Have fun