1) In SP, switch to Trevor. Then, go to a public session.
2) Withdraw all your money from bank to cash and and deposit back at least $185k to the bank.
3) Click any vehicle over $185k on the Warstock website (it may also work with other sites), and hover over "BUY IT NOW".
4) Press START and X as quick as possible right after each other. It can help to hold START and let it go right before you click X.
5) You should see the big map in pause menu now, highlight "Leave GTA Online" press START once and you should see this alert message "You have insufficient funds. You can add GTA Dollars to your in-game account. Would you like to visit PS store?" That means you did step 3-4 right.
6) Press START and X/A as quick as possible right after each other. Then quickly press (R1, X, UP, X, X), so that you leave GTA Online (if you don't see Leave GTA Online, repeat step 1-5).
7) PSN/XBL Store will load up while you are leaving GTA Online. When you get to SP, press O to back out of it.
8) After SP has finished loading, Warstock will pop up automatically and you will see Cargobob price as $2.2 million.
9) Click the home button on the website and re-enter Warstock, and you will see the Cargobob price as $185k. Buy it and you're done.