Need a little help: to do with paypal

Ok so I want to buy a few items from walmart.com but due to chargebacks from selling gold paypal has put all my money I recieve on hold for up to 21 days. they have a bunch of little technicalities that can change the period of time but nothing was clear on this.

If I buy a green dot moneypak card from walmart and I use it to fund my paypal account will everything be instant or will it be on hold I am sure with all the buyers\sellers on this site someone should have my answer.

Also how does it work as in do I go to the store tell them I want it or pick it up take it to the clerk and have them add x amount? I was a little confused on this aspect if anyone can help me out I'd really appreciate it.