iSS does not impose any income limits, nor is engaged in the payment process - there are shoppers who earn dozens of thousands of dollars via iSS and use shopping as a primary source of income. It's important to note that iSS is just a platform where you can find shops and fill out shop reports - you are still performing them for the specific MSPs that post them, and those MSPs are paying you.
Each country has its own taxation laws and ways of processing taxes. If you're in Canada, it would be suggested to consult a tax attorney in Canada, and vice versa if you are in the United States. If you are a shopper in both countries, solicit attorneys in both countries. Can't hurt, as long as there's a free consultation. In my opinion, hope this helps.