PAYGO $1.99MB 3G HNDST356 means that you were accessing either Get it now or Mobile web from your 3G handset.
Any time you transmit data you are charged, and your total usage is rounded up to the nearest MB, so if you use 5KB of data, you will be charged $1.99 for 1MB of data. Simply accidentally clicking on the "Get it Now" or "Mobile Web" functions, no matter how quickly you cancel, will incur this data charge.
PAYGO $1.99MB 3G HNDST356 means you "pay as you use" data on your device which is $1.99 per megabyte. Instead of having a data package on your account, you chose to pay as you use data. Hope this helps!