Historically I've thought AvaTax should not be internet based; rather for performance reasons and classic internet reasons (no internet, slow internet, loss of internet, things that @BethBowers endures), it makes more sense to have their app directly installed on a client's network where it would periodically download latest sales tax rates.
But then I learned this:
1) At least part of their pricing model is charging by the number of invoices you post (you are not charged for the sales tax calc done in S/O Entry that is ""free""). So they can easily capture the volume of invoices that way and charge ""per transaction"". It's also true the higher the volume of invoices the lesser the per invoice fee.
If it wasn't internet based and entirely subscription based (presumably initial fee + monthly fee + M&S) , the up front ""visible"" charges would be higher and the service would appear more expensive to the client, whereas with per transaction model, client sees lower up front costs but would need to estimate the real cost of the service based on projected volume. The client may also see it favorably as a service that grows with them, that on a good month they have more services fees, on a bad month they have less service fees.
2) Some of the reports and services they provide is based on the captured data from the posted invoices.
So being internet based has its advantages maybe that's why SpeedTax went that route too.