In Canada, I am about to send a 2-pack of my product, and I have placed them in plastic bags measuring 4 x 6 x 6.5 inches.
Do I need to put a suffocation warning label on them?
Yes, polybag opening of 6 inches or more requires a suffocation warning label.
Yes, Canada requires a suffocation warning label on plastic bags with any opening ≥5 inches, and your 4x6x6.5" bag qualifies so add the English or French warning.
Yes. Amazon requires a suffocation warning for poly bags with an opening of 5 inches (12.7 cm) or more.
Since your bag is about 6.5 inches, adding a suffocation warning label is the safer choice to stay compliant.