Hi Jim,
Toolbars are stored in *.xlb files. According to my Walkenbach book, for non network users this will be in Windows' main directory. The book also notes that a toolbar that's attached to a workbook automatically appears when the workbook is opened, unless the workspace already has a toolbar by the same name. Sounds like what you're experiencing.
Maybe your client needs a one-off deletion of the old toolbar so the new one that you sent with the workbook takes effect. I think this can be simply done from your clients machine in the VBE's immediate window, Application.CommandBars("The Toolbar Name").Delete
If is it not convenient to open the VBE from Excel on the client's machine, you could email a temporary file with that line of code to execute when the workbook opens & then close/delete that temporary file. Open the other file with the new toolbar and it should be OK.
Normally when using custom toolbars in a spreadsheet, standard practice is to (On Error Resume Next) delete the custom toolbar when the workbook opens (just in case it was already there), then create the toolbar on the fly, hide the toolbar whenever the workbook is deactivated, unhide when activated, delete when the workbook closes.
HTH,
Fazza