Domestic flights for foreigner in Colombia

Yes, you can book via a global service like Expedia or Hipmunk, but the rumor you've heard is also correct: you can get significantly cheaper seats by booking on a Colombian site.

For example, if you visit avianca.com (the international site) and search for flights from Cartagena to Bogota on the randomly chosen date of February 7th, the cheapest flight on offer is US$87:

But if you visit the local avianca.com.co and do exactly the same search, the cheapest flight is COP 128,600, or only US$45 -- that's 48% cheaper!

For avoidance of doubt, you can book these fares outside Colombia and they're available to all who know where to find them. (This is different from countries like Peru, where "resident fares" require local ID and the ID will be checked before they let you board.)

Please don't overthink this or worry about it so much.

For Colombia, there's really nothing stopping you from buying a ticket Cartagena-Bogota from any regular travel web site, Expedia, Kayak, etc.

If you want to price shop, see what price they give, then try to book the exact same flight at the airline's website.

Finally, 'foreign transaction fees' are usually related to the payment method. Even if you do have one, they are typically very small.

You can easily book a one way ticket from Cartagena to Bogota on any reputable US website, such as www.hipmunk.com. LATAM and Avian offering 10+ non-stop flights per day and it's around US $80 for a one way ticket (late January, mid week). LATAM can be as cheap as $63.

I don't think it gets any easier or cheaper than that. What would be wrong with such a solution and what exactly is the problem you are trying to solve ?