...on-line)? A?A discount, it can save your some money directly.
B?A membership, you can receive more after-sale service.
C?A free gift, such as a special package etc.
D?Two-for-one offer, buy one get one free.
A: the discount makes me feel like I am getting a deal, but I'm not signing my life away or having to buy more than I want. But for you, if it's your company, I'd do a membership so that I could keep one-time customers updated and send them emails with future sales and stuff. Most of your business will probably be from repeat clients.