Zenram, one thing to consider in this instance is that the actual vendor of the product is RPGNow / DriveThruRPG - not you - and they'll typically handle the sales tax.
However, as mentioned, tax is a very tricky subject. Talk to the guys at RPGNow to make sure everything is good. It might be a good idea to talk to your local tax authority, as well, rather than laymen. For me, that's the County Tax Assessor and Collector. The appropriate office for you can usually direct you to the right person to ask. See if your city / county / region has a government website to get you pointed in the right direction.