Bahn afaik isn't allowed to sell you tickets when the start and destination are within a regional operator, and my regional operator (to get to work) doesn't have a web shop.
The bus drivers are comically confused when I try to buy "einen Viererkahrte zu Aachen, drei Stück" so that I don't have to deal with it every other office day (this I can pay with a convenient 50€ bill and lasts about a month, give or take). Then I still have to stamp them per trip and will be last in line for a seat, but at least I don't have to bother the driver most days.
The DB app allows you to buy tickets for local transport associations - not sure if it includes everything, but so far I always found the regional tickets there.
The legal difference is that here the DB (app) only acts as proxy, i.e. you don't buy a DB ticket for the local transport but a local transport ticket through the DB (app).
On the other hand if you get a DB ticket across regions it might include local transportation at the begin/destination of the cross-regional route as part of the DB ticket. And this they are not allowed to provide you if your DB ticket isn't cross regional. E.g. if you buy a ICE ticket from one station in Berlin to another station in Berlin they can't offer that. (Most ICEs passing through Berlin stop at least at 2 stations in Berlin).
When I book a train ticket, the regional trains are already included. My local public transport system (HVV) has online & smart phone ticketing for many many years now.
A regional operator where the ticket can't be bought online or via a smartphone should be really really rare in Germany by now. Which operator is it, which has no online payment option?
AVV.de, the menu options for ticket buying are "am Automat" and "Vorverkaufstellen" which is a list of physical stores.
...which I guess is why I never looked further and saw they have an app now. Never heard of Naveo before, maybe it was introduced recently? Either way, that looks like it might now be possible! Why did no driver ever mention / did the buses/stations not advertise this before it's ripe for being replaced with the 50 euro subscription?!
Well, you could of course use some tickets from other operators in the same region. In NRW there is the handy eezy NRW[0] ticket system where you just pay the kilometers travelled "how the bird flies". So even the connections that would be expensive otherwise could be cheap.
I use it sometimes for the last bit of my long distance travel if I visit my parents (not directly in VRR region but in Regionalverkehr Münsterland (RMV)). It states that it works for all stops within NRW - which is annoying as one of the busses I could take actually goes to Osnabrück (Niedersachsen) and I cannot use - even though it's actually operated by the NRW regional transit provider.
Overall public transit is / was a mess in germany and 9€ Ticket really was an eye opener on how easy it could be to hop on and off of everything that's moving however you like.
The bus drivers are comically confused when I try to buy "einen Viererkahrte zu Aachen, drei Stück" so that I don't have to deal with it every other office day (this I can pay with a convenient 50€ bill and lasts about a month, give or take). Then I still have to stamp them per trip and will be last in line for a seat, but at least I don't have to bother the driver most days.