I definitely think the quick shipment is the most important part of making people happy. When someone sends you money, they are in essence trusting you to now deliver what you said you would. Every day that goes by is another day of waiting ... wondering ... worrying. Will the package come? Will it be what they want? Will it be broken?
So the more quickly you can get that package to them, the less stressful it is for them, the happier they are and the more likely they'll tell people about you and trust you for their own future purchases!
I've been including little origami creatures in the CDs I've been selling the past few weeks. The recipients really seem to like them <img src="/images/graemlins/smile.gif" alt="" />