Try offering an affiliate program if you haven't already. It can increase your sales drastically and you only pay for the results.
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One of the biggest obstacles was that I hated all of the affiliate apps on Shopify. They all require all your customer data (email, names, shipping addresses), and most of them have privacy policies that grant themselves very broad permissions with user data.
I refused to give up our customer data like that, and so we rolled our own by hand, but it's very manual, so the only way for affiliates to find out their earnings is when I manually run a script and pay them once a month.