I got help from Paypal tech support (who knew there were such nice, patient, helpful folks at Paypal?!!).

The tag at the end of the link is for the Total Due (it's irrelevant whether you use Unformatted, Column Aligned, or Non-Column Aligned or whether you place it in the Header, Footer, or Invoice Item Info sections). That tag inserts a space between the dollar sign and the first digit of the money amount. That space breaks the link and keeps the Total Due from being "seen" or inserted at Paypal.

At this point, the only solution I know of is to open the queued email and remove that space before sending it. As far as I can tell, this works nicely.

Does anyone know if there is a way to alter the Item Total tag to prevent it from inserting that space?

Thanks.