You could use a Search repository or Find Entries activity to get the invoices. Then you can loop through them with a For Each Entry activity, get the multi-value field and use token functions to sum the values. You could then use Assign Tokens to create a new multi-value token and append this total to it. After you've gone through all the documents, you would have a multi-value token of totals for all invoices. You could then sum these values for the overall total.