Michelle,that is considered to be spamming - you cannot do that. Many customers do not leave feedback, probably only 1/2 of my sales have feedback.
One way to get some is send a thank you note with your shipment and say at the end "If you are pleased with this transaction, could you please leave Feedback". Or, if they send me a personal email how much they love the item, I email them back, thank them and ask if they can leave feedback.