Even though it would technically be a requirement to amend the 2017 tax year return to report your 2017 RedBubble earnings for the amount involved it should be safe to include it as a lump sum in 2018. It may also be that your expenses for creating this income would render your net profit as zero and therefore not taxable. In which case not reporting it would be satisfactory to CRA. This does not apply to all self employment income but small amounts that do not involve net profits can be ignored as they are considered as hobby income.
