Getting enough money in this borderlands is very anoying, since unlike BL2 or 3, level cap is at 40 and prices are stuck at 5-15k per item. Getting last upgrade for the bank costs 18 000 000 gold. So, for this case, my save does have inventory to sell and thus have enough money for this last upgrade..

BUT! As some of you pointed out, my save file is a bit faulty - there isnt enough money to get bank upgrade. I think I know the reason for it - I used save editor to raise bank upgrade lvl and I guess it didnt translate into save itself. You can download save editor here: https://github.com/somefunguy/TTWLSaveEditor to raise bank upgrade lvl to 22/23 and buy last one to get the achievement. A bit anoying, I know..

Save file location: C:\Users\YOURUSERNAME\Documents\My Games\Tiny Tina's Wonderlands\Saved\SaveGames

Before doing so, back up your own save files.

IMPORTANT NOTE -  This save file is 100% save file, meaning I already have all other achievements. This means by using this save, all other achievements will unlock automatically (thanks MillBridge101 for this information). Some games do that (other example I know of is Generation Zero). Just be aware, if this achievement is literally last one you need, go ahead. If you wanna unlock rest by yourself, my save can wait.