This is one of the easiest achievements in the game, all you require is 25 items to put in to the safe. This includes every item such as; Medical kits, Any ammo, and evens guns.
The easiest way is to pick up as much things as possible and then go to the nearest store, move the things you do not need in to the safe. Make sure you do not transfer everything over, because keeping certain things is essential in case you need them. If you keep doing this then you should eventually get the achievement.
Another way to do this is to save up quite a bit of money, then transfer all your current items in to the safe and with the money you have saved up; buy medic packs and transfer them over. This is the way that i gained the achievement. After you have the achievement you can sell all the medic packs back so it does not waste money. Even if it did, it would still count towards the achievement for spending 300,000 credits.
I CAN ALSO CONFIRM THAT ALL THE ITEMS DO NOT HAVE TO BE TRANSFERED OVER TO THE SAFE AT ONE TIME. ITEMS CAN BE MOVED OVER THROUGHOUT DIFFERENT PERIODS OF TIME AND WILL STILL COUNT.
As long as there are 25 items in the safe at once, it will unlock.