Ask for the referral...

Referrals and word of mouth advertising are some of the best converting business opportunities you can have. As long as customers are happy with you, referrals convert better, buy more, and buy more often than random people you do business with because they’re powered by word of mouth and trust.

So in each confirmation message after purchase ASK...
So in each email as a link ASK...

Ask as frequently as you reasonably can... it works!