Maybe you can open the .csv file in one worksheet.
Then split it up based on the customer (if you have a dedicated column for
customer).
You may want to look at the way Ron de Bruin and Debra Dalgleish approached it:
Ron de Bruin's EasyFilter addin:
http://www.rondebruin.nl/easyfilter.htm
Code from Debra Dalgleish's site:
http://www.contextures.com/excelfiles.html
Create New Sheets from Filtered List -- uses an Advanced Filter to create
separate sheet of orders for each sales rep visible in a filtered list; macro
automates the filter. AdvFilterRepFiltered.xls 35 kb
Update Sheets from Master -- uses an Advanced Filter to send data from
Master sheet to individual worksheets -- replaces old data with current.
AdvFilterCity.xls 55 kb
If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm