Sometimes my customers don't receive their product delivery emails because it ends up in a gmail Spam or Promotions folder. How do I fix this?

Consider putting a note on the thank you page after they purchase. Be honest & say something like "Be sure to check your inbox for your purchase. Please be advised that there are times when it may also end up in your Promo/Spam folder depending on your email provider.'

Also, evaluate the email that you're sending. It may have too many links and things like that that's causing it to end up in the promo folder.