Many different countries and cities now have low emissions / environmental zones, and most use a windscreen sticker. There is not one scheme for all of Europe, not even for multiple countries in the EU - more is the shame, it would be so much easier if they did..
Germany has the umweltplakette (mine is green), France has the Crit-Aire (mine is yellow for the same car), Austria has one but not for cars yet (but does require the Vignette for driving on Autobahn), Slovenia has both, Switzerland only requires the Autobahn Vignette. Some cities in Belgium require registration but do not require a sticker - including Antwerpen where I will be headed soon.
It is important to ensure that you have the right equipment (the required safety equipment is different from country to country), have the correct permissions to drive in each country (as I mentioned above), understand the differences in driving law in each country - for example you don't say which country you are from, but if you are not an EU resident, driving in Austria, France and Italy you are required to have an IDP in addition to your home driving license - and each country will have different speed limits based on the type of road and whether you are in a built up area: you will only see a "National Speed Limit" sign and have to know what that means in each country.
I am really surprised that Avis has a no-drop-charge plan between Switzerland and Munich. I've got to believe that they are sticking you somewhere for it - I've learned over the years that there is no such thing as a free lunch. Read the fine print Very carefully. If so, you are the first person I have seen here in 10 years or so writing on these Forums and previously Helpline to have that. Congratulations if it is true. That can really help others.