Create a List view with a Sub Summary Report and have the Customer field
Sort your records on Customer field
Create a new table for Customers and have a ID Unique number for each Customer. Then import Customer from your file and then match those Customer to your portal
Your question is slightly confusing. Johan has described how to do what you ask for, which is how to see the total invoice value for a Customer. But I am guessing that you are using the 'Find' function of Filemaker to just find one Customer's invoice lines, and then you want to see the totals of each invoice.
If that is the case, do as Johan suggests, but create a sub-summary part on your layout and define it as 'When sorted by InvoiceNumber'. Create a Summary field = Total Of Invoice Line Value and pout it in the sub-summary part. Sort the records by InvoiceNumber.
Maybe even better, create 2 sub-summary parts, the upper one 'When sorted by InvoiceNumber', and the lower 'When sorted by CustomerID'. Then sort by CustomerID, InvoiceNumber, and you will see the invoices grouped by Customer (and their total) and each Invoice for that customer totalled too.