Car will not start without a jump

I just had a new battery installed in my car because I had to keep getting jumps to get it started. I was able to drive to and from the mechanics, but the next day when I went to start my car it would'nt start. I just got a sort of knocking or clicking noise. The mechanic said that the starter and electrical was all fine at the time he did the repair, so what could be the problem now? I was told to tow the car back to them and if it's the battery they will pay for the tow and fix it at their expence but I am afraid they might say it's more than that and will be charged a lot of money for something else. What do I do? Also I purchased some cheap engine fluid (or dry gas) could that be the problem and is there something I can use to flush that out?