Simply go to Moxxi's bar in Sanctuary and there is a tip jar on the bar where you can tip her $1000 at a time. Put $10k in the tip jar and pop! trophy earned
Here are a couple friendly tips -
1- Upgrade your backpack storage at blackmarket (crazy earl) as soon as you have enough eridium to do so. Then pick up everything you can to sell
2- If you are short on cash and dont want to blow all of your money, just tip her the $10,000, wait for the trophy to pop (DONT MOVE!) and simply dashboard and close app. when you restart you will still have the $10k spending money
3- If you have plenty of money and wouldnt mind getting something in return for tipping her, continue giving her money even after the initial trophy pops. After roughly $30k, you will get a red text blue rarity smg or shotgun. Tip yet another $10-$20k and receive the other gun
HAPPY HUNTING AND GOOD LUCK!