First trip to Berlin. Will be there 6 days and will travel to Zone C 1st & last days (BER) as well as a trip to Pottsdam. Looked up 6-day WelcomeCard with Zones ABC on Expedia. All cards on Expedia that include Zones ABC say “Valid for 48 hours after first use.” Really? 48 hours on a 6-day card? Is that an Expedia thing or is that the way WelcomeCards work?
I don't imagine there is any advantage to buying a Welcome Card from Expedia rather than from the official website, which is https://www.berlin-welcomecard.de/en
As is the case in most if not all major cities, Berlin's transit authorities sells its own single- and multi-day transportation passes. You don't have to buy the Welcome Card to have an all-you-can-eat transit pass.
“Valid for 48 hours to 6 days” You choose the validity when you select the number of days you want.