roaming in USA cant I just buy a data package

Quick question, I just wanted to check something with you all about roaming in USA.  With the other providers i have used in the past i always buy a "data package", e.g 10gb for £20.

Am i right in thinking that Smarty dont so that, they ask you to add £5,10,20 "out of plan" add on, then you used data, calls etc at 10p meg until you run out of money.

Is that right? there is no "data package" i can buy.  

Just want to make sure i buy the right package before i get a load of roaming charges.