For some reason it's not listed on the ADP website, but there is a decent sized Galeries Lafayettes in S3, and they definitely do Handbags (I have the credit card slips as a result of Mrs jsfr's last trip as proof).

Not sure about Longchamps, but they did have Lancel and Tumi in December.

As far as the price is concerned - I find them all rather expensive and can recommend some great places in Dubai, Beijing and Hong Kong where you'll definitely pay a lot less (as long as you're not caught by customs)!