Personally, I upload to a specific gallery on my website. Give them the address and tell them how to find their gallery. They can view proofs online and order whatever they choose right there. Zenfolio (through MPIX) prints and sends them the photos and credits me for the profit (since I markup the photos for the customer purchases).
Everybody is different, but I haven't had any complaints about doing it this way. It makes it easy for everybody to see their photos and even for their friends and family to see the photos and order some if they want. Makes it really easy on me too.