Really long game problem!!!?

ok so i went to gamestop and bought this game ive been wanting for a long time, Tomb Raider Underworld, and i got it for my ps2 then i played it when i got home but then it wouldnt save, at first i thought it was the game so i went and exchanged it and i tryed to play it again and it still didnt save then i thought it was the ps2 so i tryed saving other games to make sure it wasnt the psp and it did save the other games so we called gamestop and they told me the number for the sony company and i called them and they said that it wasnt the ps2 so we tryed calling the company that made the game and they didnt answer. it was to late to return the game at gamestop. what should i do? will they still give me store credit? or will they let me trade it in for another game? plzzz help