For scannable coupons the most important thing is crisp edges on the bars and text... Use a printer that lays pigment black on plain paper and set the driver to plain paper normal quality. High quality modes dump more ink and can feather on cheap sheets. No more misreads.
Paper choice is simple. Bright white 24 lb around high 90s brightness gives good contrast and smooth feeding. If you go thicker, make sure your printer supports that thickness and use a straight rear feed if available.
