Looking to set up a referral system (think something like "invite a friend and get your next order free!") where a customer enters a friend's e-mail or has a unique code. I've found a few options, but they have very expensive monthly fees.
If you prefer a one-time charge for the full campaign, Maître offers a $79/campaign package.
I used Maitre. Super simple to install as it's a widget that can be embedded on any web page. The design/font is adapted from the block you put the code into, however, you can customise most of the stuff from your Maitre dashboard. It also integrates natively with MailChimp and supports webhooks & callbacks, so that you can really customise it they way you want.