If you're in the US or Canada, Avalara is a solid solution. They offer sales tax calculation and filing (in some states) and integrate into the popular shopping carts/accounting software. The agency I work at has done numerous implementations for large online retailers. Last I checked pricing was at $350/yr for up to 700 transactions.
I might be an old fashioned Canadian, but I refuse to use online services for my taxes because I don't know where, or with whom that information is being hosted. So I buy a disc or download the software every year to do the taxes and I file them by sending them over the net to the CRA.