EDIT: I forgot to mention this - if you're interested, there have been books written about this topic. I highly recommend Bill Norton's The Internet Peering Playbook. Available in print or digital copies - it's pretty much the de facto text on this kind of stuff.
DISCLAIMER: The examples used here are hypothetical only - I have zero insight into the business relationships of the carriers mentioned in the below post.
The Internet is really controlled by the peering coordinators of the Tier 1 ISP's around the world.
In the Internet connectivity business world, there's transit and peering. There are also various forms of peering (paid peering, settlement-free peering, etc) but for the most part, these are what Internet connectivity as a whole boils down to. Yes, it is complex because when it comes down to it, there is a cost involved with getting your bits from Oklahoma to Germany and back. It's not just technical jargon and routing policy - there are significant economic and business drivers involved as well.
The term "buying transit" just refers to a contract where you're paying a provider money to get bits off of your network and on to someone else's network. Keeping to your example, let's say there are three providers involved with you sending traffic from your local office in Oklahoma to a branch office in Germany. Let's say your local provider is Hurricane Electric. Hurricane Electric gets the traffic to Germany via Level3. Level3 connects to a major German incumbent provider - DTAG, aka Deutsche Telekom AG. DTAG will then get your traffic to its final destination if the local office is buying transit from DTAG, or from someone else that relies on DTAG. So the connectivity would go something like this:
Local office -> HE -> Level3 -> DTAG -> local German ISP -> remote office
HE is not a tier one carrier, so they likely will buy transit from Level3, or may have paid peering with Level3. Level3 and DTAG are both tier one carriers and thus are most certainly peers, and the local German ISP that your remote office connects to would likely buy transit from DTAG.
Forgetting the routing involved and only thinking about the money, one needs to understand traffic ratios. Typically the big tier one carriers (like Level3) will not peer (referring to settlement-free peering here) with others unless the traffic ratios are somewhat even. Unless you're another big carrier, this will almost certainly never be the case. Most local ISP's and content providers are called "eyeball customers", because their ratios are outbound-heavy. Taking that into a business context, Level3's network would be utilized much more than the content provider or local ISP's network. At this point, you're right - Level3 does not benefit from peering with a local provider or a content provider, and it simply wouldn't happen.
However, if you are a very large ISP with a great deal of customers and also a good chunk of originated prefixes, but you and Level3's traffic ratios just aren't quite there yet, then paid peering may be an option - often much cheaper than buying transit.
Now what if you're not a big carrier? Do you benefit from peering with other people, even if they aren't carriers or ISP's? Yes. When you buy transit, you're usually paying for a set cost per megabit, billed at 95% percentile usage rates, with a minimum commitment. If you're a content provider, it's in your best interest to peer with as many people as possible, because this just brings the cost per megabit of your transit down, because when you peer with folks, you have a direct route to their network, and you no longer need to rely on transit for your bits to reach their network.
Also playing into all of this is routing policy of these tier 1 carriers. Typically these carriers will do hot-potato routing. What that means is if Level3 peers with DTAG in multiple geographic locations, Level3 will likely dump off the traffic destined to your remote office at a DTAG peering point as quickly as possible, rather than carrying the traffic on its own network to a DTAG peering point that is closest to the final destination.
You also run into situations where carriers neglect peering ports in order to strong-arm their peers into doing something they want. In other situations in peering relationships, one carrier will simply de-peer with the other carrier, causing devastating impact for a significant portion of Internet users (see Level3 and Cogent).
To answer the original question - the TL;DR answer is all networks involved with getting the traffic from the source to the destination, pay in some way or another. The longer answer is that if you're going to buy transit, make sure you're buying the transit from a reputable carrier. You're not just paying for uptime and availability, you're also paying for the carrier's connectivity as well.
