Every order you fulfill is an opportunity to get a word of mouth referral. It’s a great moment: The customer just opened the box, they’re excited about their new thing, and they are happy that you sent it to them.
This is when you ask for the referral.
Put something in every single box that 1) asks someone to make a referral and 2) makes it easy.
Include something that is easy to pass along: a postcard, an extra catalog, a discount coupon, stickers, a free gift, or an extra copy for a friend.