I purchased train tickets online. Several days later I went back online just to make sure our reserved seats were no longer available for purchase.

I noticed that the price was now less expensive than when I purchased our tickets. So I wrote them an email but didn't expect anything to come of it. After all, prices changes all the time.

Well, as a "gesture of goodwill" they sent me a vouvher (code number). This voucher is good for use is several ways. Among them is bahn.com, etc.

Is it possible to purchase tickets for Koblenz and/or Heidelberg online with no fixed date? Or at least a range of dates (such as between a date and x number of days following?

You can probably use it at the station in Koblenz - no advantage to booking online - flat fare on regional trains anyway.

Actually I'd probably take IC trains as the regional trains are much slower.

I think i have it figured out.

Tickets good for any day are much more expensive. Tickets that can be used on that day (which ever day I pick) but on any train are a few Euros more.

When I buy the tickets the time we go I'll probably know ahead of time. However, I'd probably want the return train time to be flexible.

So I think I'm getting the hang of this.

Thanks