greenhouse newbie

I have always wanted a greenhouse.  I don't know why I think it stems from when I was a kid in Washington state and my mom's friend had one, and it was so cool and I loved to go in it.  My B-day is the last day of July.  My daughter was asking me what I wanted and I told her Sam's club had a 4'X4' greenhouse on clearance for 40$  Now this greenhouse doesn't even compare to the one I remember, or all the cool ones on this site.  But I was very happy with it.  I live in N. California where the growing season is very long.  There isn't really a need for a greenhouse so I wanted to start small and see if I would use it.  I finally put it together.  It is light pipe held together with pvc connectors and a plastic shell.  There are 4 shelves on 2 sides.  The door is two zippers that can be rolled up.  I am using it to start veggie seeds.  We have been having unseasonable hot weather mid to high 70's  When I go in after noon, it is very hot, I worried to hot for the seedlings, so I left the door unzipped and pulled back on one side to let so air in and now I have a greenhouse full of fly's. (I hate fly's)  There is no other way to let air in.  Should I not open the door, live with the fly's?  Help.

I have read this post a few times. I can only come up with 3 realistic options.  The first is to see if there is anything beyond the warmth that
Is attracting the flies. The second is  go to a farm store and buy a fly trap like one that might be used around livestock.  The final one is that my experience has been that Hornets love to feast  on flies. But that could leave you with another problem.

I just deal with the flies, since I have been opening the door every day, I only seem to get the tiny ones.  The new issue is to much sun.  It is literally frying my little seedlings.  I put cardboard in a few places so there is still lots of light, but less direct sun.  This has made a huge difference.  It makes me glad I went with an inexpensive GH to begin with so I can work out all the bugs before go to the trouble and expense of building a real on.  You just can't only rely on what you read.  Most every post said to put the GH where it would get full sun all day, but in my climate, at least for starting seeds that is just not true.  I don't know if I will ever build a greenhouse, I don't really need one.  Maybe some day I will, mostly because it's a project I would love to do, more than need.