Always disclose your affiliate relationship. Most visitors will probably understand that graphic ad will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.
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.
Cost per action/sale methods require that referred visitors do more than visit the advertiser's website before the affiliate receives a commission. The advertiser must convert that visitor first. It is in the best interest of the affiliate to send the most closely targeted traffic to the advertiser as possible to increase the chance of a conversion. The risk and loss are shared between the affiliate and the advertiser.