I've used Stripe for payments and they have an option to collect sales tax, for example, but it's still on you to register and remit those taxes to the jurisdictions where you sell.
What I'm looking for is a payment service that collects payments from abroad and takes care of all the tax compliance and just does a payment to you as a US to US company so you only have to report one source on income on your taxes. Similar to affiliate marketing where you advertise to an audience all over the world and when they click through your links the business you market for takes care of all the payments and taxes and just sends you a direct lump commission .
Does a service like this exist?
If it's a SaaS this applies. For mere licensing of software I don't believe it is clear cut, and maybe if it's not "mandatory" they might want it for tax streamlining.
For EU you certainly don't have to register it on every country, an invoice from one country is accepted in all countries.
Easiest way? Sell it through a 'store' like Windows Store, Mac Store etc. Not sure there's something like Steam for 'serious software'.