Tax and Shipping API plugins
Craft Commerce is a young product and as such, there are indeed limited options for third party plugins.

Good news, though - writing a shipping plugin at least is not difficult (well, the Commerce part isn't - the API of the shipping service is of course, up to that shipping service!). See https://craftcommerce.com/docs/shipping-methods to get started.  I've personally written a live rates plugin (for TNT) and several unique table based ones, and I've found the process easy and clean.

There is already a ShipStation plugin - https://onedesigncompany.com/craft-cms/plugins/oneshipstation - and ShipStation handles UPS, FedEx etc.

On tax, I don't know a lot about that and so far I think Commerce's tax support is a little limited (e.g. no tax on shipping support).

Fortunately, Craft/Commerce support is excellent and if you do need to integrate a new service, you'll fine Luke Holder and the rest of the Craft team are very helpful with that sort of thing - and of course they're keen to see the Craft/Commerce eco-system grow.  support@craftcommerce.com is the address you need :)