I am not a pool dealer, just a noob myself who is in research phase before I buy. I was told Lomar and GLI are great liner companies. If I were you I would NOT buy another liner from that same store. Go find the thickest liner you can buy online, I think at least 25 gauge, they may make thicker. You will have to spend a little extra for quality. I see some that carry a 15 yr. warranty. Good luck!! You sound like you have my luck, LOL!!
I agree with you on this. Best to choose a 25 gauge liner. You'll have to pay a bit more, but guaranteed that this investment will save you money in the long run. Some liners even have 25 years of warranty. Although, I wouldn't attach too much value to this number. Pool liner warranties are usually pro rata. When you need to replace your liner, the manufacturer will only reimburse the "current" value.
Personally I like the Stoney Creek 3D liner. It's a full print, 25 gauge liner and very good quality for its price. It should be available for 18' round pools like yours. Hope this helps.