Oh, super handy! Thanks for asking... I was playing with the SSRS end of this... Getting the count of selected values is easy in SSRS. It's the passing that back to SQL Server that could be an issue.

return list of comma-separated values: JOIN(Parameters!Invoices.Value,",")
return a count of items selected: Parameters!Invoices.Count

Just wonder if I can pass them back to the stored procedure now.. (do I need a TVP?, or can I do it an easier way?)