Hey Linda...so I realized that I had this guide in my little pocket bag for SXM. All I had to do was text this code once we landed. Didn't have to bother with the store or Mailbox visit, and I was surfing and using data right away. I do have a lot of credit on my TopUp card and also in the phone's balance (#191#) you can also check the data balance, but they expire (*91).
I just texted 3GVP 10D to 3434 and it took $16.75 from the phone's balance and I got 2.5GB
Instructions here (sorry, you will have to rotate to see):