CMYK vs. RGB
If you want the colors on your printed mailer to be as accurate as possible, you will need to use a CMYK Color Palette.
To make a long story short, the total number of colors that are available for print are less than what a digital screen can render.
To make a long story short, the total number of colors that are available for print are less than what a digital screen can render.
If you designed a mailer in Canva, using any of the available RGB colors, those colors may or may not be accurately represented when the product is ultimately printed.