This can be attained after leaving the planet Pandora and boarding the ship Sanctuary.
To gain access to Moxxi's Bar you will need to complete a few tasks once you get on board after which speaking to Ellie allows you into the bar.
Once there simply go to the jar at the counter and you will be given the option of tipping $100 (Press X) or $1,000 (Press Y). You can tip either of these amounts and the achievement will unlock.