Fun fact: the “Amazon Associates” program actually has a different program for a variety of different countries, meaning yes, Amazon.COM has a different program than Amazon.CA, and Amazon.CO.UK, and Amazon.FR, etc. etc. If you want to, you can sign up for all of them without being residents of these countries. What’s important is where your readers are from.
In addition to Amazon’s clear commitment to conversion rates (read: probable sh*tloads of testing), the extreme growth of Amazon Prime. Estimates from top firms calculated Amazon has between 65 and 80 million prime subscribers in early 2017, which is up from about 54 million just a year earlier. The huge boost in Prime subscriptions has apparently had a massive impact on conversions.
A Direct Message on Instagram is only slightly better. It can get lost in the mix of other DMs, and it’s often a crap shoot as to whether they will even read it since if they don’t follow you on Instagram, your DM will show up as a “message request” in their inbox. These are often just ignored. However, Influencers on the micro-level are more likely to check these DMs so for those with smaller followings, it’s worth a shot.