Tiny gap between the lid and base of my closed MacBook Air

Methinks you make much ado over nothing.

However, you you have just purchased this MacBook directly from Apple, then you have a fourteen day window within which you can return it for refund or exchange without question.

Otherwise, if you feel strongly that this seemingly very minor issue must be addressed, you can arrange an appointment at Apple to have it evaluated.

You can make a Genius Bar appointment for hardware/software service and support using this link: Contact Apple Support - Apple Support

Call Customer Support (800) MY–APPLE (800–692–7753) or on line /getsupport.apple.com/ 

or call AppleCare Support at 1-800-APLCARE (800-275-2273) 

Things I would check were this mine:

— Look for keyboard marks on the display. That suggests the center of the "lid" has been bent in, likely in an overly full backup or messenger bag. The display assembly is quite thin.

Make sure the battery is not swollen. Symptoms:

— Unexpected trackpad and/or keyboard behavior.

— The gap between the display and the deck is not even when the display is closed.

— The gap between the bottom plate and chassis is uneven in places.

— Some of the screws that hold the bottom plate have popped out.

— The trackpad surface is higher than the surrounding deck surface.

— The computer "rocks" a bit on a known flat surface†.

† — I find solid-surface and granite countertops very effective flat test surfaces.

Is the lid with the screen swollen or the base? I am unable to see with the pic clearly what is posted by you.