2006 nissan altima 2.4

My car started having issues that I troubleshot down to the sending unit. Literally like 1 day before I was gonna buy and change it out myself, my car died and it was from lack of fuel from the sending unit. I had to have it towed to the nearest shop that could work on it, so I did.  Seemed a little shady of a shop but I didn't really have any options at this point. They checked it out and came to the same conclusion about it being the sending unit. After they changed it, it took a while to realize (maybe a few weeks or a little longer) but my gas gauge wasn't reading correctly after they changed the sending unit. I didn't even notice till I ran to about a quarter of a tank left and my car died. It was out of gas. Then I started noticing sometimes it would read one level then all of a sudden it was way lower. It took me long enough to find out that I doubt they will take any credit for doing anything to have broken it or had an issue during the work so, I'm just wondering what u think it could be and if it's gonna cost me another sending unit or if it's something I can troubleshoot and maybe repair myself. Thank you!