2003 Ford Expedition I have to jump my car most of the time

I am having a problem with my vehicle. I thought that maybe it was the battery but I got one of those free battery checks at Auto Zone and it turns out that that is not the issue at hand.
Let me explain briefly what is going on. Sometimes when I go to start my truck it just clicks, Sometimes It will make a sound like it's going to turn over but it doesn't and then it clicks and then other times it will start over and over again for up to a week before the issue will once again arrive.
I have to always keep jumper cables on me and back into every parking space at a store just in case it will not start and I can easily be accessed to receive a jump.
I have no idea what's going on and My husband is baffled as well. I'll tell you one thing tho, between having 4 children, one on the way and scraping by on a budget, we can't afford to take it in for a check up.
I would appreciate anything that you can tell me at this point. I am at a loss and to top it all off, the car is not even yet payed for.
Thank you so much for your time.