Years ago, I was a contractor for one of those big box stores. That $299 install had to cover: 1) coming out to the site to measure / inspect the job. 2) order the door / install materials. 3) wait, then wait some more for the materials to come in. 4) get the call that materials came in, then schedule an appointment time with the customer. 5) go to the store to pick up the materials (if your lucky everything will be there, if not.... start over with step 2 again). 6) go install the new door (hopefully no unforeseen problems occur). 7) get rid of scrap materials.
Then consider that the $299 is what the customer gets charged.... The contractor may see $175--200 of that charge.
So, the contractor has to pay for his time, gas, truck maint., insurance, etc...
All said and done, he might make $75-100 for the job. Some jobs were very simple and easy to do... some were nothing but trouble, so the profit goes way down quickly. The contractor is getting paid 'flat rate'.
I did not do it full time, only "filling in" between real jobs, just to keep busy (why not?).... Well, after 6 months, it really wasn't worth it (Profit wise).
These days, I wouldn't trust any of the installers working for the big box stores.....
Do it yourself for sure!