Email's not really for sending large files. How big are they? And how many customers?
You could try what is done on UseNet: split them up into multipart .rar archives, send them part by part, and then have the receivers reassemble upon receipt. But it's going to be some extra work on the clients' part... not to mention extra opportunity for error on the part of the customers (I'm assuming you want this as as dummy-proof for your end-users as possible). Rebuilding multipart rars can be a pain in the neck, especially if all parts don't arrive.
As for Demz's suggestion of FTP, that's an idea,
but sending large files over FTP can be impractical, especially if you're on an unreliable connection (what happens if your FTP session terminates at 98%?). Using stock FTP - unencrypted, over WAN links - is also, by definition, insecure.
Actually, here's a better link:
If the content you need to send is sensitive, make sure whatever transfer method you decide on is encrypted.