The cost to upgrade to a pro printer is so minimal I just can’t fathom why anyone would get walmart/costco prints. As a pro, I stopped offering high res files to portrait clients as they print them at costco and look terrible. Seriously, many people say they look “fine” or “good” but if you look at them next to a pro print there’s no comparison. Spend the extra $.10 for a 4×6 or $.50 for an 8×10 and use Nations Photo Lab, or Bay Photo (depending on which side of the country you are on), you won’t regret it.
Oh, and word of advice if you don’t think it’s a really good image: keep prints small they look better that way. That’s why so many photos look great on Instagram… :)