The downside is that Shopify is only appealing for people who have physical or digital products to sell and have a need to set up a Shopify store, including site hosting, payment processing, and all the other services offered by Shopify. This can significantly narrow the appeal for this affiliate program. But if you can distinguish yourself by educating people on how to use Shopify, how it can benefit their business, and/or make them money, you could potentially big money via the affiliate program. Add in the 2 x monthly fee commission rate, and landing just a few sales of their mid-tier and top-tier products can result in significant earnings.
Stand your ground if the kids misbehave. Be firm with the kids. If they try to tell you that their parents let them do something that the parents specifically said not to do (like stay up past their bedtime), err on the side of caution and obey the parents’ rules. The parents will be impressed by your maturity and responsibility and hopefully invite you back!
Retain some control. If you upload photos of yourself, or friends/family with consent, it's worth going for the 'rights managed' licence option – otherwise you'll have little to no control over how your images are used (eg, you could star in an ad for haemorrhoid cream). See Alamy's page on understanding stock image licensing for more on the different types of licences.
Some advertisers offer multi-tier programs that distribute commission into a hierarchical referral network of sign-ups and sub-partners. In practical terms, publisher "A" signs up to the program with an advertiser and gets rewarded for the agreed activity conducted by a referred visitor. If publisher "A" attracts publishers "B" and "C" to sign up for the same program using his sign-up code, all future activities performed by publishers "B" and "C" will result in additional commission (at a lower rate) for publisher "A".